I'm 24 next month.
Probably need to find someone nearby who has a similar sized 400 and see what I think. I have quite short legs which might help.
I'd probably only be using it for the odd Friday commute and Friday/Saturday afternoon blasts around the countryside so I don't bankrupt myself in the M3. I'm on a hoon tank at the moment and it's been floating between 18 and 23mpg for the last 200 miles
A 400 would probably suit me better than a bigger bike in terms of how it's likely to ride. I don't really want to be on something that is retardedly fast in all honesty.