Hmmm... being a 27y/o not sure how valid my comments are, as 40-50 seems a way off yet!
I think it all depends upon what you want from your 'lee - if all you're after is a "no-strings" tickling session or two, then as long as both consent, fine. If you're at a stage in your life where you're looking to develop a more longterm "relationship" with someone who's interested in/willing to participate in tickling, I would question how much outside of tickling you would have in common with someone 20-30 years younger.
This topic's got me thinking, actually - at this point in my life, I wouldn't have a problem tickling an 18y/o - ok, so there's only 9 years difference. But that said, I really don't have much interest at all in tickling someone who's more than a year or two older than me. I don't know - I guess at the end of the day it's all down to personal preferences!!