In the UK, skydiving is covered by the British Parachute Association Operations manual. This covers many many aspects of the sport, but to name a few:
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You have to be aged between 16 and 18 to begin skydiving in most countries. There is usually a weight limit of 100kg's to do a tandem parachute jump, although this is less is you are doing an accelerated freefall (AFF) course. You height and weight for and AFF course must be proportional.
If you are aged 40 or over, you will need a medical form signing by your doctor. Generally speaking there is no upper maximum age limit.
As regards disabilities, that is not necessarily a hindrance to skydiving. There are many cases of para and quadriplegics who make tandem skydives. There are some conditions which wills top a jump though - narcolepsy, epilepsy, some forms of Diabetes, head neck and back injuries.
There are hardly no rules for base jumping just the fact that you have space to land and all the equictment then it's fine.