BETTER TO APPLY AS A MINOR OR WAIT AND APPLY AS AN ADULT

I'm in the processing of assisting my Costa Rican boyfriend in applying for a TRV. At present he is 17 but will be 18 in July. My father (Canadian Citizen) will be sending him a letter of invitation, and will accept full guardianship and financial responsibilities once he is here.
My only concern is if it would be easier to obtain a TRV while he is a minor or an adult?

P.D. I already have his mother's support and agreement that she will fill out the necessary paperwork if we decided to apply while he is a minor. (he does not know the identity of his father)

I was also wondering if presenting proof of my personally accompanying (via roundtrip air ticket) him back to Costa Rica would be sufficient to demonstrate that my boyfriend would not stay in Canada.