I don't know the soonest any ROTC scholarship triggers DODMERB. However, I do know that I applied for NROTC in May and I got my DODMERB exam a couple days ago.
I don't have this problem but I'm just curious: What's the big deal if a woman doesn't have a period and she wants to serve? I find it it rather sexist to put all these strict regulations on women wanting to get in when men don't have to worry about whether or not they have a period or breast...