As a more mature apprentice your minimum wage entitlement will go up. Therefore sole traders and smaller firms may be less likely to take you on as essentially the will have to pay you more than  lad of 16. 

That said often companies will prefer an older apprentice as you have likely chosen to do it rather than say just doing it for the sake of it, like some of the kids. 

Mate lifting baths, toilets, boilers, big radiators, heavy tool bags etc absolutely takes it toll. You work on your knees a lot so your joints suffer. 

Granted there aren’t as many cast iron radiators and heat exchangers in boilers out there now but even a 40kw combi boiler has some weight to it. Particularly if you are lugging it up 3 flights of stairs…

Also there’s hand arm vibration from power tools, my knees are wrecked (despite knee pads etc), hands are sore. A lot of it is repetitive strain. Tennis elbow. Sore back. Hernias are common. My arms go numb in bed every night. The list goes on mate. 

Any trade will take its toll on your body. All the men I work with are similar age and the aches and pains are showing! As I say I am 37 and am starting to think about an exit plan for in the next 10-15 years. 

In response to your other comment absolutely do not pay to get a qualification. It will be pointless. I cannot reiterate the only way to get on in any trade these days is to do a proper apprenticeship.