It depends. Take your lead from the batsmen that face you. If they struggle, hop around and back away then that is plenty pace enough.
The quickest 13 yr old Ive ever seen, was in a game I umpired and he amazed everyone there. The boy was lightening.
We put a decent estimate (though you can never be specific) of around 125 kph (approx 77 mph).
He took a wicket with his first ball, but finished with 3 overs 1 for 36 in his first spell.
A combination of a coach and captain not being able to set the correct field (he had 5 slips and 2 gullys) and facing a quality bat that wasnt overly scared of him and played through the line.
High pace at 13 is great for scaring averagep layers but I wouldnt worry too much about being top bracket as good players will still find way to score runs.
The flip side is that although this kid went at around 12 an over, all everyone could do was rave about him and talk about him for the next week or 2. There is no doubt fast bowling is ***y