Personally I think 20 is a *bit* old for a 17 year old, but not too old... I say go for it, at least for a while, to see how it works out!
I may be biased though because there's some really awkward age differences around where I live... hell, I know a guy who's 18 and dates an 14 year old! And he's not even unattractive! (lol).
I think when you're above 17-18 then the age differences can be a bit higher than when you're younger... and then when you're 20 it's okay to have large age differences, but I'd still find 10-20 years a bit inappropiate. Over 30 I'd say age doesn't matter anymore (both parts beeing over 30, that is... hehe)