@The Pentacle Queen,
I think I've remained pretty ambitious. Heck I am currently writing a book at age 41 which at this point I think could actually be published. (I've had illustrations published and some writing published at a minor level, but not a novel.)
Note, I'm not necessarily thinking this WILL happen, and in fact think that it's more unlikely than likely, but I'm putting some effort towards that goal and am ambitious enough to think it's a possibility. I'm not assuming that it won't happen so I don't even try -- I'm trying.
I do think that there are generally more options and possibilities for young people than for older people, just as you move along in life and obtain more responsibilities. (Partner, children, house, career, etc.)
That's not written in stone, though, and is not purely a function of age. I know a woman who is in her late 60's and just started a new career. She's a widow, her parents have died, and she never had any kids, and she has lots of options and is doing a lot with them.
She also has enough money, though, as do I, and that has a big impact on ambition in my experience. There were periods where I had very little money and the energy that now goes towards Big Ideas had to go towards the more mundane stuff of keeping myself fed, clothed, housed, and emotionally stable.