the problem is that any promising 18 yr olds are expensive and if you train them 2 seasons, they are 20, and a lot less worth, so the increase in price you get from the skill-ups, might be less then the decrease of the age... that is where you need luck.
If you buy players with less potential, they are less expensive, but you also get much less from them once they have been skilled-up since the buyer also knows he will not be able to improve him any further... so I'm not sure if this works all the time.
TL is too unpredictable to be able to say there is a system which will be profitable.
Some systems will be, others will not, and anything in between. I honestly don't dare to say which are and which aren't at this time.
From hattrick it is so rusted that it works that way, but please do take the test: write down the cost of your player, calcualte the amount of wage he has cost byt he time you sell him, and see how much you get, then have a look at what you gained, taking into account the player was, or wasn't usefull for your teams' performances during that time... I think many would be surprised.
They are not your friends; they dispise you. I am the only one you can count on. Trust me.