If you grade a trade, say tiling from 1-10 then anyone who is half decent with there hands can get on to the first level (1) on a week course, a few years pass and hopefully you have learned 2, 3 and 4 without making any career threatening mistakes, another few years and you could get to level 5 or 6 IF you try new things and develope your skill set, which will be joinery and plastering whilst improving the finer details of your tiling, you should be constanntly asking how you can make your own work better. By the time you have been doing all of the above for many years doing different types of tiling, (ie, bespoke high end tiling or low end graft working as part of a team) then you could go on to say level 7+... who knows, i been doing now for 6 yr and still find it as hard as i did when started... one thing changes though, you tile quicker and the results get sharper.
So whats the easiest trade to learn? depends on who is learning, I never seem to stop learning tiing but thts because im pasionate about it, someone else could have stop learning years ago!