I hope the answer isn't 40.
Seriously tho, I don't think there is a "too old" for any hobby. It's a matter of enjoying it. Once toys stop making you happy, it's time to stop. I like TFs because they engage my mind- I love modding them to work and look better, and I love analyzing their designs and marveling at the transformations. (And making pew-pew sounds once in a while.)
I hope I never "outgrow" that.