My son has a June birthday and is currently on target to graduate right before his 18th birthday. My girl's birthdays are over the winter and they would then graduate at 17.5.
I personally am trying in some ways to get away from a traditional school year. I feel bound by it. So far, my kids have been able to move up in level with no problem each year. My youngest this year is doing Little Hearts but last year she did Beyond with her big brother so I bumped her down for the year. Next year she will start Bigger at 7.
For both of my girls, school has come more easily. However, my son is very different. I can see huge improvements this year in him and that growing tremendously in so many new skills that I am absolutely amazed at his stamina and abilities.
Today he had to copy a Bible verse, draw a rooster and a hen from one of his books and then copy a paragraph the hens. He did the whole assignment alone and was very excited to show it to me when he finished. My son has auditory processing disorder. For him, some things don't come so easily. Reading was a HUGE struggle and burden until recently. He is finally reading very well for his age, planning to start DITHOR by March (I am letting him and my youngest finish a few choice books they picked a while back before we jump into formal literature study). For him, I do think that as he gets into the more independent guides, he is going to have a harder time with completing work independently. He is doing very well this year in Bigger with the things he is supposed to do but he also has issues with fine motor skills. I am trying to take a step back this year and give him space and time.
The only thing I anticipate holding him back is that he still frustrates very easily still. For example, I tried to start dictation recently with him. He got confused when I said one of the words and he immediately shut down. He took his pencil and chucked it across the room. Then he jumped up, tears rolling down his face, ran screaming to the playroom and threw himself down behind the couch. Needless to say, I gave him some space and let him cool off for a good 10 minutes. Praise the Lord that He has finally given me a good spirit to laugh when sometimes I want to cry LOL. Will my son be frustrated that he can't graduate with his friends.....well, maybe. For us, we don't really tell the kids what grade they are in. We just tell them that they need to finish each level and when they are done, then they are done! He doesn't seem to care too much that he isn't on the youngest age range at all. I don't know that he will care if he is on the oldest either. The only thing I must avoid at ALL costs is that his younger sister and him are not combine! If that happens, heaven help us! My youngest is so fast at everything - math, reading, spelling, memorization, she pretty much puts me, dh and the other 2 kids to shame. God made her great at book learning
And while my son doesn't resent her at this point for how easily school comes to her, he will if they are combine in a guide. I don't know that I think it will ever become an issue either because my youngest is 20 months younger than my son.
But back to your question about his thoughts on his friends graduation and not him, most boys I know that are the same age as him are a grade below. If I had sent him to school, I would have held him back a year. I guess for me, I feel like I have a year to play with already. For his sake though, I know in my heart that God uniquely knit him together and he is a little bit higher maintenance than my other kids. But some of the gifts God blessed him with steal my heart! While describing his actions above is sometimes a standard reaction of his, he does this because he struggles with using language and words to express his emotions (what boy doesn't, right?). As he gets older, he will continue to increase his vocabulary and maturity will naturally develop within him. He is already night and day different from when he was younger - these types of outburst occurred almost hourly. They are now every few days. My goal as his mother is to continue to help him grow into the Godly man that the Lord has called him to be. And for him, if takes graduating a little bit later than his peers, so be it
I definitely don't think you NEED to complete every level of HOD to graduate your kids. RevtoRev and MTMM are very easy to modify and make high school worthy. For us, if things reveal that my son would be better off modifying a lower level to make it high school worthy and not complete the last level, then we may do that. At this point though, I don't think it will be an issue. I personally would only hold my son back from graduation more so because of the emotional need, not an intellectual one if that makes sense.