ive been building a fast training squad (lvl 3) and my advice is to buy high value players in age 19 or 20.

Yes, they wont be that long term because once they enter age 22 they will train slower but personally i wouldnt keep them for more than 3 years myself. and they actually cost a lot more in age 18. proof? i bought a 18yo who is worth 3.28M and i thot well he must be super fast trainer. turned out he is fast, but a bit slower than a same quality 20yo i bought for 1.38M. yeah i couldnt believe it, and did a few more tests. same result. im a non token buyer so i saved my reaources and bought 3 20yo fast trainers instead to harvest off their progress. my team now gains 16 sp for a real match, pretty decent for a non-token buyer i suppose.