Actually, there's no reason Doc can't pay him normally. Keep in mind Macaulay Culkin was what, ten when he did Home Alone? Acting is very much a job, but being a minor, there was a limit to how long he could work in a day, one or the other parent had to be present at all times, and the money went to the parents (who wound up spending it all, but that's an aside. )
However, as a minor, there are of course restrictions- his parents would likely need to sign any official paperwork, plus likely something saying they agree to let him work, etc.
And there's a limit to how long a child/minor can work in a day- I'm not sure, and it might vary by area, but I think it's like four hours per day, and might be a limited number of days per week or pay period.
And there'd be some loopholes to sort out with things like insurance (IE, would Doc's insurance cover a minor) and possibly even things like OSHA (considering there's any number of dangerous items and activities going on down there.)
I've never tried to hire anyone, especially not a kid, so I'm kind of guessing there. But really, if Doc (and/or Sandy) were willing to run the right kind of paperwork, there's no reason I can think of that Jinx couldn't work at least a few hours a day at the shop.
The "Can you be 18 on a tax form?" thing was basically a way of saying "we could save a lot of paperwork hassles if you could suddenly be 18 on some forms."