Go for it! I'm 14 and I generally do the Times 'Difficult' in about 7 or 8 mins, which probably relates to your 'Medium Hard'. I find Su Doku relaxing but also mentally stretching, as I push myself to get faster each time.
Give yourself a target, like try to get the time down to under 6 minutes 5 times in a row, then get it down to under 5 minutes 5 times in a row.
I'd also like to see a forum for junior players appearing soon, as I have seen more and more appearing on this site in the past few months. If there is a Junior Su Doku book, why not a Junior Forum? (Hint hint, Pappocom)
George