There is no such thing as a window of opportunity. The only guide is when desire and opportunity coincide, then go for it. It can happen at any age.
About joining a fraternity, I would do a lot of research before going that route. You need to decide if a rich-boy party frat is for you, or the more community minded, networking type of organization.
As for getting married and having kids, again, desire and opportunity can come together when you least expect it, or never. Don’t be too concerned about the details, but rather enjoy your life, and when/if the time comes make sure you have a lot to share with a partner.