Hi Julie,
There is no upper age limit for claiming carer's allowance and it doesn't affect your DLA. However, it can affect other benefits that you or your mum might be getting, such as income support and it overlaps with some benefits.
So, it's vital that you get advice about both your own benefits and your mum's before deciding whether to make a claim.
You can read more about carers's allowance here:
Carer's Allowance Good luck,
Steve