I won't argue about the games played vs innings pitched here, just wanted to chime in on where I start my prospects. For me, it is when do I forecast their need on my BL roster. If the players truly are legit #1's, I would think about starting them in HighA this year. So their progression is half a season in HiA, 1 in AA, 1 in AAA. By then they are probably BL capable, and will still continue to progress at the ML level to their final potential.
If I am set at the ML level at the position, and it is an 18 year old, I may start them at rookie and let them progress 1 level at a time just because there is no rush.