Of course it is OK to have a couple of old guys on your roster. I have my trainees, my core players, and then I fill the last couple of spots with some cheaper old guys. However when I buy those old players I consider that money gone. If I spend $1 mil on a 23 year old guard, in several season of use I can still sell him for close to what I got him for, maybe 600-800k at 27 or 28. I got several years good use, but I got most of my investment back, too. He is still an asset. Old guys aren't.
Training is very important. It is too big a subject to talk about here, but don't take training lightly. Get a training plan going, don't just wing it. It is better to focus on a few guys and really train them up than to think you can slowly train the whole team. Young guys train, middle age guys play, old guys fill last needed spots. Read about training.
I notice some of you have what I call 'Jordan-itis'. All you care about is scoring. You want to look at the box score and see a bunch of points. Great, wonderful, awesome, bad plan. Buying and training nothing but the scoring skills will end up getting you nothing more than high scoring losses. Defense matters, more than offense overall (imo). If we both started with the exact same teams in every way, and you trained offensive categories for half a season and I trained defensive categories the same time, I am confident I will beat you 8 out of ten times. Defense really matters.
Secondary skills matter, a lot. Skills like passing (the most underrated, imo. All my guys can pass at least some) handling, driving and rebounding matter. Rebounding on non big men is especially helpful. The engine rewards more balanced team rebounding over 1 or 2 high skill rebounding bigs and a bunch of small men with atrocious RB. If you are buying a guard, and he has the skills you want plus say Average RB, he is so much the better, you will out rebound other teams eventually because you have SOME rb on all of your guys. Also, RB is very expensive on bigs. Look at the transfer market and the prices of similar skilled guys with the only difference being RB. At high end (like Wondrous and above) RB gets costly, be careful. If you are in the market for a new player, especially an expensive game changer, take your time, shop around, get the right guy even if it takes a little longer. Hasty purchases can set you back financially a long time.
Anyway, good luck, was a fun couple of seasons, hope to see some of you on the court soon.