I think the grandparents are in their mid-80s. It’s odd because as you note they had his dad when they were teens and he is only 20. But maybe his dad waited until late in life to reproduce or skateboard boy was part of a second family. Also might explain why dad died of diabetes, you don’t see that often with young men and he didn’t look obese in the pics.
Debbie looks pretty old as well, like she might have had Colt at 45. Unless the desert sun is unforgiving or she is a smoker.