From a MAG perspective, Jeff Robinson, head of the Men's JO program here in the states, warned about getting little guys into grips too soon.
If they're swing sucks mechanically and we put them into grips and they swing harder and bigger with poor form, we are asking for injuries since things are out of alignment. We can also be encouraging poor grip strength before swinging big and using grips as a crutch.
I have one friend who occasionally will hop on apparatus and still swing giants on high bar, front or back, without any big deal. However, he is 5'3 and maybe 145. I don't see a guy 5'8 165 or 170 doing this when it comes to the big moves.
As for the girls, I think too many of them, especially when they are itty-bitty sized and still are too weak to swing right are getting grips too soon. Once their form is decent and they are being limited from their swing creating too much force for their grip strength ( the other route would be going into extreme grip training, being able to grip 3-5 times their BW in say a DL or MA similar grip training ).
Many other countries don't use grips simply because of the expense. Some use homemade tape grips or the fact comes they are teeny tiny besides being elite strength in shape.
Say we have a teenage girl, hitting 5'5" going though compulsory or L7/prep-opt. She is really going to tear her hands up besides just creating a lot of force through her swing.
Grips make release moves much easier.
Gals like Sacramone are a lot bigger than Shawn Johnson and Liukin or any of the chinese/romanian gals. Or most elite gals period.
Getting back to it, I'd say get the girls into grips come L6 when they start learning giants or swinging big. Most L5 do not swing big enough to need grips. Nothing in their routine requires it.
As for the guys, many could do everything in L5 without grips. It could go either way. I wouldn't start doing big swings on rings without grips or giants.