What? shorter distances are perfect for younger athletes to target. Using argument that some east Africans that dominate the marathon now at a younger age (some now in 20s instead of mid or late 30s)is NOT a good comparison. They typically start running at an early age to build a huge aerobic base (run to school,then as they get older track practice on top of that). No US kid does that. Our society isn't set up that way. Shorter events are a good place to start.
The OP thinking about different training ideas is great. Let him build a mileage base, then in 10 years can figure out best distance. He has to run them all at a good fitness level anyways to find out.
You're not done and past your prime in your 30s. Especially if you haven't started until 15-18. Case in point, how old are Kimetto and Kipchoge - yeah, 30.