it's about cat's?

They've obviously stopped grooming - but you have to ask why. If it's just in certain areas (like their back) it may be because they're getting a bit less flexible and can't reach. If it's all over, then they may have dental problems so take them to the vet for a check-up. Cats don't stop grooming unless there's a reason.

You should have addressed this before the fur got so matted it needs shaving.

Its coming from when your cats are grooming themselves or each other and their fur gets wet and thus how it get clumped and matted. Yes, take them to the groomers or something to be shaved and once their fur starts to grow back, continue to comb them that way this wont happen again. As far as them being clean cats before, perhaps due to their age they are starting to neglect on their grooming that is certainly possible.

Its old age. The same way people get old and don't take care of themselves in the same way. Old men will stop shaving or coming their hair, and old women will stop wearing make up or jewlry..

Cats get old too and don't take the same care of themselves. This is wear you come in. Brush their fur out every day if they'll let you. Older people need caregivers. Older cats do too.

as cats get older it becomes harder for them to groom themselves

Yes, your statement is about cats.

They don't groom themselves as much.