All bowling balls should be cleaned after each use. This can be done by using any cleaning agent available from your favorite pro shop, or using a commercially available cleaning agent that will not leave a residue on, or damage, the ball. All balls pick up dust, oil, and other foreign substances as they roll down the lanes and up the ball return equipment. This can alter the ball's reaction by hardening or plugging the pores and on the surface. By keeping the ball clean, the ball will react more like when it was new.
If you use a dull sanded ball, and with normal cleaning the ball doesn't react as well as it used to, it is time to sand the ball. This will bring it back close to its out of box condition and reaction.
As a bowling ball goes down the lane and it crosses a seam, a small nick may be caused on the surface of the ball. Once there are enough of these little nicks to cause a loss of reaction, it is time to resurface the ball. This can be done to any ball, including polished balls. They are sanded smooth again and then refinished to their box condition. Plastic balls should be polished often to keep them working as they should. These can also be resurfaced when the need arises. Your favorite pro shop should do sanding and resurfacing so it is done properly.
Mass bias is something that can be found in any bowling ball, but has more meaning in today's equipment. It is a heavy area on one side of a ball's weight block/core. This is created by either moving the weight block/core to create a pin out ball, by using an asymmetrical weight block/core (that has some sort of a protrusion). There are many theories as to how much effect mass bias has on a ball. If the ball does not have one of the latter two weight block/core types, the mass bias will be very minimal. The two key elements as to how a ball will react are the type of coverstock (and its finish), and the weight block/core location in relation to the ball's track.
It is always best for the bowler to be able to use a ball drilled for them. It really doesn't matter if they are still growing. A bowling ball can be plugged and redrilled several times before it needs to be discarded. Even as the hand changes, the weight of the ball may remain ideal for the child for 1-2 refits. Weight does not become a factor until they start to grow and get stronger. With the cost of the type of equipment a youth bowler would use, it isn't a bad idea to purchase a new ball. Even when the child grows out of it, you can always hand it down to a younger generation of bowling youth.
A custom thumb is a general term for a thumb insert that can be repeatedly made for size and pitch and can be installed in any bowling ball. There are several different manufacturers and systems available that create thumb inserts (we use Exact Thumbs from Vise Grip). What you need to do is determine which of your ball's thumb holes feels the best. Take that ball to your favorite pro shop (if they have the capability to make a custom thumb for you). A silicone mold is made of that thumb hole. This takes 24 hours. Then an insert can be formed from this mold. This can take anywhere from 30 minutes to a few hours. This insert can now be installed into a bowling ball. Using the same mold, several inserts can be made so they can also be installed in all of your equipment and have the same feeling thumb hole from ball to ball. The original ball that the thumb mold was derived from will remain unharmed and legal.
While we do have the ability to install "interchangeable" thumb inserts, we do not fully support their use. While these have been successfully used by many bowlers for many years, our experience shows that the need to have two (or more) different sized thumb inserts can nearly always be negated with proper fitting and drilling that allows for proper grip and release. Also, there is no way to guarantee that the various locking mechanisms do not break - if this occurs during a tournament or league, there is typically no way to continue to use the damaged insert and/or ball. As noted above, we are able to exactly reproduce custom thumb molds (inserts) that allow for same feel to be achieved in several balls.