Have no idea, but I think they would softer falls, but if i had a hard fall, I would change backprotector. If, per chance, you dont fall off, then I would continue to use it, until it takes a heavy fall!!!!!
Body protectors should be replaced at least every three to five years, after which the impact absorption properties of the foam may have started to decline.
Garments bearing the now obsolete Level 5 or 7 labels will no longer be effective and should be replaced with a garment bearing the current BETA 2000 and 2009 Standard.
If you should have a heavy fall, your body protector should be checked immediately for dents. The foam will expand back to its original shape within 30 minutes; but if a dent is showing on examination, then its likely that this part of the garment has lost its impact absorption properties and should be replaced.
Hidden damage that a body protector may have sustained is also a good reason for avoiding second-hand garments or those with unknown histories. Taking good care of your body protector means that it will last longer within the three to five year recommended lifespan. So dont leave it lying around at the yard or in the lorry to be chewed and trodden on. Spending too much time in a hot car or damp tackroom wont do it any good either.
When not in use, body protectors should be hung on a clothes hanger. Doing up zips ensures they keep their shape, while closing Velcro fastenings will help prevent them becoming clogged with horse hair and hay particles.
Most body protectors are made from heat sensitive PVC nitrile foam, which is why they feel increasingly comfortable as they soften and mould to the wearers body. So try storing your protector in a warm (but not artificially hot) environment to ensure its nice and flexible for that early morning ride.
When I'm feeling a bit more flush I'm replacing mine as soon as I can - the cover is giving way so the foam no doubt has moved/is moving and I had my heaviest fall in it last September so I would say it's on borrowed time and local tack shop agree as it's probably 7 years old and been worn every time I ride.