I was involved in a startup in my 40s. It ultimately failed, but I learned lessons that will hopefully be valuable to you to. What you describe sounds like a dream job for most people. As long as you get it, I don't think you have to be concerned at all about being older than the others. They will appreciate the times when someone comes up with a bad idea that looks good, but you can say "I've seen this before, here's what happened..." - as long as you are right. Even better will be the times when someone has an unproven idea and you can say, "I remember a couple of times when one of our developers had an off the wall idea that we all wondered about, but it was appealing enough that we went with it anyway and it worked." As for the hours, there will be 20 and 30 somethings who will go on 24+ hour coding binges. Did you do that when you were in your 20s? Do you think you would be productive doing it? Does management expect you to disrupt your family life? It's hard to believe a company that has grown to have 300 employees would have leadership that expects all of their employees to destroy their personal life. If they do, the company won't be the success everyone hopes for anyway. (Well, the founders might walk away with a lot of money before it implodes, but you won't. You have to assess that risk.)
The great thing about a good startup is the chance it offers to to new kinds of work and see it succeed in the marketplace. This can be really exciting. It's possible that you might have a similar opportunity in a large company but the odds are very low since you will be separated from the product or service by layers of management and bureaucratic rules. Yes you will get a steady paycheck, but it will never compare with the huge win you can get at a startup and the satisfaction of knowing you had a direct role in the success. You can also ask yourself if the startup role will make you a better developer. If the company fails, will you have improved your technical knowledge so that you are still valuable to other companies? In an established company it's more likely that you will just be a code monkey whose skills slowly evaporate without you realizing it, although you don't sound like the kind of person who would let that happen. If OTOH, the company you work for is run by PHBs who are forcing you to work on obsolete stuff, you have to leave anyway. Some large companies do have great jobs, though, but I don't think you would be looking if you were really happy where you are.
From your description of the job and given that you don't sound like the Get Off My Lawn type, I would suggest that you join the startup if they make you an offer that is reasonable.