In The Fellowship of the Ring, Bilbo was celebrated his 111th birthday. Then he gave the Ring to his nephew (Frodo Baggins), after being confronted confronted by his friend Gandalf the Grey. How old was Frodo at that time ?
Remember that they "summed up 144 years" and that it was also the majority birthday for Frodo. This is all explicit in the book (no English quotable text at hand, mine is a translation).
"Bilbo was going to be eleventy-one, 111, a rather curious number and a very respectable age for a hobbit (the Old Took himself had only reached 130); and Frodo was going to be thirty-three, 33) an important number: the date of his 'coming of age'." (From book 1 chapter 1)
The combined ages of Bilbo and Frodo were 144, therefore Frodo would have been 33 (they shared the same birthday, hence Bilbo referring to "Our Birthday.") As Einer mentions this is a dozen dozen, in English this is a gross, but the word gross can also mean very rude or coarse, vulgar; when used as an adjective. Thus some of the Hobbits taking offence at being referred to as a gross.
The invitations were limited to twelve dozen (a number also called by the hobbits one Gross, though the word was not considered proper to use of people); and the guests were selected from all the families to which Bilbo and Frodo were related, with the addition of a few special unrelated friends (such as Gandalf).
The Lord of the Rings Book One, Chapter 1: A Long-expected Party