It depends on price mainly

Compare your 18 year old price to other 18 year olds of the same quality, which is more? The more expensive one trains faster. As for your 25 year olds, they may be lower quality so they can train at an average pace. There is also speculation that 18-20 year olds training speeds have been slightly adjusted so that they don't become overpowered, they would regain faster training once they become 20-21 though.