Age
We’re never told about the Colonel’s age explicitly, but Mrs. Bennet does seem to refer to him when discussing the merits of one of her daughters marrying “smart young colonels.” Context clues suggest his wife is quite young, though that doesn't necessarily prevent the Colonel from being significantly older.
Family and Situation
At the beginning of the novel, it would seem Colonel Forster is either completely single or simply engaged. He gets married somewhere between Christmas and Easter, bringing Mrs. Forster back to Meryton with him. Later, they move to Brighton together.
Colonel Forster is primarily responsible for a regiment which includes Denny, Chamberlayne, Captain Carter, Pratt, and later George Wickham. One of these gentlemen is likely a Major and the Colonel’s second-in-command, though we’re never told who that would be. Towards the beginning of the novel, the regiment encamps at Meryton, later removing to Brightonfor the summer.
It’s likely that the Colonel employs a servant named Sally in his home at Brighton, if not elsewhere, based on the content of the note that Lydia leaves for his wife.
Character
Mr. Bennet evaluates that Col. Forster is a “sensible man,” and thus trusts him to look after his daughter Lydia. Given how poorly that situation goes and how generally silly we're told Mrs. Forster is, this may be an oversight on Mr. Bennet’s part. We are told explicitly that the Colonel apologizes to the Bennets personally for what happens and is one of the first people to start following Wickham’s trail to figure out where he went.
