Ted is the only day 1 character who's age is never mentioned.

Mary's age is mentioned regularly. She is 37 in the final season.

Lou's age was mentioned a few times in the earlier years. The last time I remember is the 4th season episode where he doesn't want a party.

Murray is 45 in season 6 when he is in love with Mary.

Also, Rhoda's & Phyllis age were mentioned.