Anne was 25 years old when she married Gilbert Blythe.
Harmon Andrews mentions Anne's age in Chapter 2 of Anne's House of Dreams.
"So Gilbert didn't go back on you after all," said Mrs. Harmon Andrews, contriving to convey an expression of surprise in her tone. "Well, the Blythes generally keep their word when they've once passed it, no matter what happens. Let me see--you're twenty-five, aren't you, Anne? When I was a girl twenty-five was the first corner. But you look quite young. Red-headed people always do."