A lot depends on his personality. Is he self-motivated? Does he have an entreprenneurial attitude? Is he organised and good with admin, planning, etc?
I've got lots of "tradesmen" clients such as plumbers, electricians, etc., and the variation in their earnings/profits is very wide indeed, even for sole traders with no staff/subbies.
The ones running it as a "proper" business typically earn twice as much as the ones who are running around chasing their tails because they're wasting so much time doing lots of quotes, forgetting to order stuff meaning time to go to find parts etc from the builders suppliers, forgetting to raise invoices/chase payment, etc., bad work planning meaning long journeys instead of scheduling jobs close to each other on the same days, etc.
I'd definitely recommend an "add on" to basic plumbing, such as GasSafe registration or specialising in heat pumps, etc - far broader range of better paid work and you're not competing with the "odd job men" type of plumbers doing the more basic pipework etc.
Eg I've got an electrician who's specialising in Solar panels and he's now making twice as much profit as he did a couple of years ago doing mostly basic electrical work. Another is an electrician/alarm installer who's moved into home "Smart" electronics, i.e. smart alarms, smart lcd lighting installations, smart appliances, etc., and he's likewise making twice the profit as he made doing normal electrical work.
You need a "niche" or specialism to make the big money if you plan on staying a sole trader, otherwise, you probably need to take on staff/subbies to get bigger jobs.
As someone said upthread, age doesn't have to be a major problem. At first during training and setting up the business, he'll have to do virtually everything, but in a few years, as he gets a bit older, he can pick and choose the jobs he'll want to do if self employed, so he can choose to only take on work that doesn't involve too much physical strain, which is where a niche or specialism comes into play. Let the "odd job men" do the jobs involving lots of crawling under floorboards or in tight loft hatches etc., or squatting under kitchen sinks, and concentrate on the jobs that you can do mostly in "normal" bodily positions, such as like for like swapping of boilers etc.