These are the right questions. People often pose the question as a matter of age, which is almost certainly not the determining factor. I will concede that there may be some loss of endurance with age that makes the startup thing harder.

I suspect that the OP just wants a bit of reassurance that he's not too old for this. Of course he's not. 38 isn't too old at all.

I'm 36, and I wouldn't try to start a startup right now. But that's mainly because I have, as you said, a pregnant wife, a three year old, and a mortgage. However, I know a few people who aren't even deterred by those things and do work for startups. I even know a founder in that situation. He is excruciatingly busy, though, and I know that it does take a toll on his personal life.

By the way, I think that working for startups and founding a startup are often completely different things. In the tech heavy bay area, I don't think that a talented programmer assumes all that much risk by joining a well funded startup, because if things head south they can often get a well paid new job within a month or two. Furthermore, "stable" jobs often aren't as stable as they seem - especially if they allow you to let your skills get rusty.

So personally, I think that working for a small startup that pays decently and gives you some nice equity isn't all that bad an idea. However, you won't (and probably shouldn't) get anywhere near the kind of equity that the folks who ate ramen and crammed into a 2br apartment for 18 months get. Instead, think of it as an exciting job with a nice potential upside (I know a few parents who scored that nice upside).