I'm 10 years older than you and worked for a variety of startups in the last 20 years, and am just starting in a "new one" yes they are all 20 years younger than I am, but ... somehow they think I can bring something to the table...
Including BTW a large network of contacts ...
About the "cultural thing", you should probably discuss with your familly and negociate that you will be very late about 4 days in each month (but not necessarely friday night)... then decide what you would like to do there night, it could be going to gothic festivals, play pool or snooker, go bowling, organize Maker's event, go to steampunk events, real time go language programming, what ever ...
Pick stuff you like and could convince "them to do", stuff they like and you could tolerate (or even enjoy) doing... things you find interesting and that you could actually organize and be seen as a kind of leader in .. (think tech oriented meetups at meetup.com or similar stuff)
Then try to find once a month an event you can tolerate and that is organized allready...
Organize once a month or every two monthes a meetup or something similar about something techy you care about and invite the rest of the company to attend
Invite everybody once a month to something you find fun (if they do not come it's their problem, just make sure it is something that they just might be interested in ...)
And think about what the fourth "free pass" in the month could be ...
nb: try to find a baby sitter if necessary for these days so that your wife can do whatever she wants also...
Even if you only organize a subset of this plan you will find out that you will have more than necessary time to "network" with the team, you do not need to go clubbing all night (you allready found your wife, and if you are thinking of cheating on her doing it in front of your colleages would be the worst possible idea...)
In office happy hours are not really a problem if you remember that you can also drink a coke without rhum, just explain if somebody is jocking about it that you are going to drive home, and they both your children and the companies investors would rather see you alive tomorrow ...
And other "week-end", holliday activities can be fun & interesting (burning man !! :)) but are not necessary, particularly if you get to speak to the same people "off road" during pub&pool or bowling or the "meetup.com about " event you organize or help organize ...
And at the end of the day, what people care the most is "code that actually works", and "answers to a couple of hard questions" ...
So don't worry if you stay cool you'll do great ....