I know in the Usssa Elite 24 a couple of years ago our players were required to use nothing less than a drop 3. Now I am not for sure whether that rule has changed but everyone on our 14u team this year used a drop 3. My opinion is to start a player using a drop 3 as soon as they can. I know if my 11 year old son who is 5' 1 85lbs can use a drop 3 as his primary bat in a game, any 14 year old player should be able too.
If a player is not use to swinging a heavier bat, once they have to pick one up it will change their swing mechanics. So why not start them as soon as they are mechanically sound enough to swing a drop 3. Just like everything else in baseball you will have to re-tweak their swing mechanics some after they start using a drop 3. One of the biggest adjustments is most players find out when they pick up a heavier bat they will have to use their legs more and stop relying on their arms as they do in the younger age groups.
I tend to agree with G-man. Both my sons used a -5 bat @ 13U and quite honestly there was no real benefit. Get a -3 and choke up if necessary or stick with a -8. The is not a great selection of -5 bats on the market and you'll use a -5 used for one season at most.