Can someone tell me if a bitch ever stops coming into season and if so at what age? 

Bitches do not go through a menopause, and she will continue to have seasons well into her dotage :D

Will they still be regular or not?

That I cant answer, all bitches are different, but generally, yes they will still be regular

Many thanks.will still have to watch the little hussy!!!!!!

Some bitches do stop or have much longer spells between seasons, particularly as they get very old, but many continue until they die.

As the bitch gets older, the chances of mammary tumours increases if she is unspayed and also the risk of pyometra increases.

I've recently had my 15 yo rescue yorkie spayed, as she had several small tumours which needed removing and, as it turned out, a prolapsing hernia to repair.  I would not have normally risked the op on a lady of her age, but as she needed one anyway, I opted for a spay.  Despite her stomach being a patchwork quilt of stitches, she was jumping around as normal the next day - but she is remarkably active for her age and seems to be able to jump onto things which are 3 times her height or more :)

Yes she has mammary tumours and also cancer of the throat so not long to go realistically. 

I'm really sorry to hear that Bella.  Frankly, the fact that she has cancer is likely to affect her seasons (similar to various illnesses in humans)and she may not have another one.

I would just let her live out the remainder of her life to the full without worrying about the seasons.

Very best wishes