I'd say that of all the people claiming to bowl above average, only about 5% actually hit the speeds they claim. We all love to think of ourselves as being quicker than we actually are but lets be honest, if most people could hit 80/90 mph then it wouldn't be such a sought after commodity in cricket.
Look at all of the true pace bowlers - how tall are they on average? I'd wager at least 6 foot. Why? Because you need to have long levers (arms/muscles) to help generate the power to propel the ball. Though it's not impossible you're very very unlikely to get anybody under the age of 14 bowling at 70 mph plus just because that they won't have the strength nor the size.
I've seen a lot of age group cricket over the years and can honestly say that in general you only ever see one, maybe two kids (out of a couple of hundred) who can touch 70mph when they bowl (and normally they're the ones who end up plagued with injuries by their late teens due to the strain they place on developing bodies in order to hit those speeds), it's not until they hit 16 that the speed really comes.
Remember, although it's great to be able to bowl fast unless you're playing top class cricket as long as you're able to put the ball on the spot 9 out of 10 times with swing and seam movement you'll always take plenty of wickets.
I bowl on average around the low 70's but that's with a good action and the fact that I have strong shoulders and long arms.