In Harry Potter and the Philosopher's Stone, Professor McGonagall is described as having black hair which would imply that she was at most middle-aged. Other characters of age were given grey or white hair. (Madam Hooch, Prof. Dumbledore)
The door swung open at once. A tall, black-haired witch in emerald-green robes stood there. She had a very stern face and Harry's first thought was that this was not someone to cross. "The firs' years, Professor McGonagall," said Hagrid.
-Harry Potter and the Philosopher's Stone
The answer to this question includes a quote by JKR from 2000 that Professor McGonagall was "a sprightly seventy."
Was she always intended to be this old (her hair color really is the only indication of a younger age) or was her age adjusted due to further character development or to reflect the casting of Maggie Smith?