45 is too old imo. My fondest memories are with my dad who was very active, doing things. Taught me how to swim, coached me through football all the way to college. Tennis, hiking, boxing, fishing... just getting up every morning and running together then working out together etc. Remembering what it felt like beating your dad at basketball after years of losing. Him being mad but proud. And now him being older, we can sit down and laugh about so much. Times when he was fukking me up and then when I started busting his ass.
You rob your kid of having those memories when you’re too old to do anything. He’s gonna have memories of a grandfather, not a father. You rob yourself of that experience as well. You need to be able to be active at least until he’s of age and on his own.