I'm 32. Do I count as young?! Love this hobby. Even my 8 month old can watch trains go around the layout for an hour at a time. Only got into it a little over a year ago, although I grew up playing with my dad's Postwar Lionel.
-Matt
I gave both sets of grandchildren (and families) Lionel LionChief sets for Christmas over the last two years. They are ALL (including the 18-month old) delighted. The youngest one- 18 months/ boy- can run the train, blow the horn and start the crew talk. He does run it pretty fast- but the thing never derails, and he laughs hysterically as the train goes chuffing around, with the horn blowing almost constantly.
I give each grandchild a Christmas car each year, with the date and "From Grandpa" written in silver Sharpie on the bottom of the car. These cars (and the train sets) will probably be with them for the rest of their life.
Giving kids the gift of a hobby, no matter how involved they get with it, is something that is an important part of being a parent/ grandparent.