I can't say much personally after the teenage years because I'm still on my late teens, but I know as a child I could lucid dream pretty frequently without trying, and I know of older members on this board who are able to lucid dream well.

The reason behind it is R.E.M. sleep (Rapid Eye Movement) in which we dream, it gets a bit shorter with age, and R.E.M. sleep as a young child is very active. Though I don't think its a big amount that decreases as we age. I'm not sure how accurite it is but this page shows graphs of diffrent stages of sleep, http://www.epilepsy.com/epilepsy/sleep_age the two graphs still confuse me, its hard to tell exactly what two age groups they are refering to in it, but my guess is people over 50 (the lower graph) and young adults (the upper graph).