I disagree that there's a specific reason for his feeling lost; this is probably due to still feeling some of that myself (I'm nearly 25), though.

That said, I joined a startup as his age. Younger, actually - I was 18. It was a good experience in that I learned a great deal of technical knowledge very quickly and it laid the path for the career that I've formed since then, but it's not a remedy for a spiritual problem.

I'll qualify my experience to say that the first startup I worked at was not a good company; our president was arguably a snake-oil salesman in a lot of ways, and myself/the other engineers often worked 'deathmarches' leading to a moment where I was literally told during a doctor's appointment that I would have a heart attack before I could legally drink if I didn't find a new job.

I don't regret the job, overall; I learned a lot professionally and personally, and the next startup was far more successful. I do regret not wandering a little more, though, and he should take this opportunity.