Why are you setting arbitrary years for deciding when it's time to get serious or marry, shouldn't that just be a natural progression when both parties feel comfortable and what not?
Lol no one here is capable of giving a decent answer to that, you've just gotta ask yourself that and him too and discuss it further, or stop thinking about it so much and give it a shot.
It's almost certainly some combination of all/most things mentioned tbh, just that there will be some factors that have a more significant impact then others.