Why do you care so much about bowling fast? The 2nd fastest bowler in my group in my club is all over the place with his accuracy - the ability to bowl at 65 mph consistently (106.2 km/h) has got him 55 wickets in 39 matches (1.41 wickets per match) in 20-over U14 games at an average of about 27 (that sounds like a good average, but with team totals around 140 it could really be better). So why do you really care about generating speed? If I were you, I would focus on accuracy and control of line and length, heavy swing at near-top speed (for you, that sounds like 59 mph or so) in both directions and possibly a bit of batsman-specific planning. I, myself, was unable to cross even 50 mph at the age of 11-12, but was still a fairly good bowler because of my unpredictable balls and my incredibly curved flights (in addition to the odd bit of ridiculous luck: for instance, I once bowled a clear wide which happened to hit the inclining grass on the leg side of the stumps; from there, it bounced off and hit the off-stump with Shane Warne-like turn (except I didn't turn it even an inch) - the umpire happened to think it had been a proper leg-spin delivery rather than a huge wide); there are many ways to be an effective fast bowler, without necessarily needing to build on speed so much. Even currently my speed is less than yours, but I still regard myself as a fair bowler (although I don't usually bowl in matches).
As for how fast you can get, I'm not sure I would say that it is down to biomechanics, although perhaps if you're a very committed and slightly above average you can make the 70 mph mark. I am always filled with wonder when I bat in nets against the top pace bowler in my group (he bowls at 75-77 mph consistently) and I often struggle to face him. That's really quite devastating speed - it's not so easy to reach it. As a batsman and someone who bowls at 53 mph usually, I wouldn't presume to give you advice about bowling faster, but I will just tell you that it's not all that important. More important is the accuracy and swing, which can trouble batsmen more than pace (for instance, I have no problem at all facing the 65 mph-bowler - he's actually easier than 3 out of the 4 other pace bowlers in my group to face despite being faster). Or, as an example which you may understand more easily, who was more destructive - Brett Lee/Shoaib Akhtar or Glenn McGrath/Wasim Akram?
I would recommend you ask your coach if you really do want to get much faster.