"many people avoid starting startups because they're afraid of seeming irresponsible."

That's a great point and I think supports another one. Getting older is generally incompatible with founding a startup. Not always, mind you.

Point #1: Ramen profitability is a higher number the older you get. People generally accumulate obligations faster than they can get rid of them (loans, kids, wives, elder parents, houses, etc)

Point #2: Yours. It doesn't look irresponsible for most 20 year olds to punt normal life and build something crazy. "It's a phase, he'll learn a lot, and then grow out of it". A 40 year old will get a lot more flak for setting aside a cushy $200k/yr job for some wild idea.

Of course, older folk who manage to live lean and don't give a crap about what people think/say will do just fine... :-)