The length of an average generation from parent to child can vary greatly from one lineage to another, so that there are many examples of families have several exceptionally long or short generations in a row. Of course, the tendency is that the greater the number of generations, the closer they will come to the average generation length.
Tang Dynasty Emperor Xuanzong (685-762) was born only 139 years before his descendant in eight generation gaps, Li Pu (824-828), crown prince of Emperor Jingzon (809-827), making an average of 17.375 years per generation gap.https://en.wikipedia.org/wiki/Chinese_emperors_family_tree_%28middle%29#Tang_and_Second_Zhou_dynasties1
Emperor Mingyaun of Northern Wei (392-423), was born only 115 years before Emperor Wen of Western Wei (507-551) an average of only 16.42 years per generation gap.
Crown Prince Touba Huang (428-451) of the Northern Wei Dynasty (grandson of Emperor Mingyaun) Dynasty was born only 79 years before his great great great grandson Emperor Wen of Western Wei (507-551), an average of 15.8 years per generation, and only 55 years before his great great grandson Emperor Xuanwu (483-515), making an average of 13.75 years per generation.
James Fitzgerald (died 1558) 14th Earl of Desmond in Ireland, married as his FIRST wife (out of four) the granddaughter of his younger bother. Thus the children of that marriage were both the grandchildren and the great great grandchildren of the 14th Earl's parents!
Similarly Grand Duke of Baden Leopold I (1790-1852) who is the ancestor of many catholic royals and nobles married his older half brother's granddaughter Sophie (1801-1865), princess of Sweden, and had children. https://en.wikipedia.org/wiki/Frederick_I,_Grand_Duke_of_Baden4
I once calculated that Prince Joseph Ferdinand of Bavaria (1692-1699), descendant of many uncle-niece marriages, was descended from Grand Duke Gediminas of Lithuania (c. 1275-1341) in both ten and fifteen generation gaps, averaging about 41.7 and 27.8 years respectively.
Sigismund II Augustus (1520-1572), King of Poland and Grand Duke of Lithuania, was descended in four generation gaps from Grand Duke Algirdis (c. 1296-1377), an average of 56 years per generation. https://en.wikipedia.org/wiki/Sigismund_II_Augustus#Ancestry8
The shorter the number of generations, the more they can deviate from the average.
King Idris of Libya (1889-1983) was the son of Muhammad al-Mahdi as-Senussi (1844-1902), the son of Muhammad ibn Ali as-Senussi (1787-1859). The average of the two generation is 51 years, but Muhammad al-Mahdi was 57 years younger than his father.
Harrison Ruffin Tyler (born 1928) is the grandson of President John Tyler (1790-1862), making the average generation 69 years, but the actual generation lengths are 63 years and 75 years.
A single generation can be much longer or shorter than the average over several generations.
Wikipedia has a long list of older mothers in the article Pregnancy over age 50. The oldest age for a women to conceive naturally is listed as 59 and the oldest age to give birth is listed as 66.
I may add that many members of European royalty are descended from Duke Leopold III of Austria (1351-1386) son of Joanna of Pfirt (c. 1300-1351), although some suspect she was not as old as reported.
In the fictional universe of Star Trek Kirayoshi O'Brian was born in 2373 http://memory-alpha.wikia.com/wiki/Kirayoshi_O%27Brien10, about 104 years after his mother's mother was born in 2269 http://memory-alpha.wikia.com/wiki/Keiko_O'Brien11, thus making two female generations averaging 52 years each.
But many fans assume that Keiko was younger, and thus that her mother gave birth to her at an older age:
It's also interesting to note that Keiko's mother was apparently around the age of 60 when she had Keiko (assuming Keiko was not much older than 40 in 2369).
If Lieutenant Uhura was about 30 in TOS she could have given birth to a child 30 years later when aged 60. That child could have been a senior staff member of the Enterprise D aged about 70 in TNG. Yeoman Janice Rand was probably younger than 30 in TOS. And then there was Yeoman Third Class Tina Lawton, aged about 17 in "Charlie X", who could have showed up as an elderly admiral in TNG, still annoyed by being called "lizard girl", and who could have had a 65 or 60 year old child on the senior staff of of the Enterprise D.
So there literally could have been a member of the female "next generation" in The Next Generation.
Of course it is much more common for men to became old parents than for women.
Marcus Porcius Cato Uticensis (95 BC-46 BC) or Cato the Younger, was the great grandson of Marcus Porcius Cato (234 BC-149 BC), Cato the Elder, making an average generation 46.33 years, which is not startling. But Cato the younger's grandfather, Marcus Porcius Cato Salonianus (born c. 154 BC) was born when Cato the Elder was 80 years old.
Massinissa (c. 238 BC-148 BC) first King of Numidia, was said to have fathered a son age 86.
King Casimir IV of Poland (1427-1492) was the son of Wladyslaw II Jagiello (c. 1352/62-1434), son of grand Duke Algirdas of Lithuania (c. 1296-1377). Thus Algirdas was about 56 to 66 when Wladyslaw II was born, and Wladyslaw II was about 65 to 75 when Casimir IV was born.
Grand Duke Svitrigaila of Lithuania (before 1370-1452) was born when his father Algirdas was about 70 years old.
I have read of two different Civil War veterans who became fathers in the 1930s when aged in their nineties.
And there are many other examples of elderly fathers not the Wikipedia List of Oldest Fathers.
The oldest on the list include Ramjit Raghav (born c. 1916) who became a father age 94 and 96, and Reverend James E. Smith who reportedly became a father aged 101 in 1951.
Ensign Chekov was 22 in the second season of TOS. If he became a father 38 years later aged 60, that child would have been 62 years old 100 years after the second season of TOS and about the time of TNG.
Captain Kirk was 34 in the second season of TOS. If he became a father 26 years later aged 60, that child would have been 74 years old 100 years after the second season of TOS and about the time of TNG. If Kirk became a father 40 years after the second season aged 74, that child would have been 60 years old 100 years after the second season of TOS and about the time of TNG.
In "Space Seed" Khan and 72 of his followers were revived (12 others died due to malfunctions) after "two hundred" years (probably almost 300 years) in suspended animation, which was probably a record.
KIRK: Is it possible they're still alive after centuries of travel?
MCCOY: It's theoretically possible. I've never heard of it being tested for this long a period.
After that example of long term success, suspended animation units for emergencies may have become more common. Thus it would have been possible for a child of one of the TOS characters to be kept in suspended animation for many decades and revived to become a member of the TNG bridge crew.
In TNG "Relics" Mr. Scott is suspended in a transporter pattern buffer for 75 years. After being rematerialized he could have joined the TNG bridge crew, though he did not.
In "The Neutral Zone" three persons frozen after death are brought back to life after about 370 years of being dead. Obviously one or more could have stayed on the Enterprise D and become major characters. It would have been equally possible for some member of the generation of the children of the TOS bridge crew to be killed, be frozen, and be revived after several decades to serve in the TNG bridge crew.
And there would have been many other methods to get someone in the generation of the children of the TOS bridge crew to become a member of the TNG bridge crew.
The creators of TNG chose how many years were between TOS and TNG. They could have chose 30 years or 300 years, 10 years or 10,000 years, as the time between the two series. And they chose the biographies of the main characters in TNG and chose not to make any of them members of the same generation as the children of the TOS heroes.
Thus it would have been plausible to make some of the STNG characters members of the "next generation" of TOS characters' families, or the same age as the children of TOS characters, if the creators of tNG had desired to. But Star Trek: The Next Generation was supposed to be the "next generation" of Star Trek TV shows, not the "next generation" of Star Trek families.