One idea is that you could follow Romeo-Juliet laws. Programmatically, it's not difficult to add such a condition to some presumed friend-finding algorithm. Forgive any formatting problems below...

if user.age < 16:
do findFriendsBetweenAges(0, 18)
else if user.age >= 16 and user.age < 18:
do findFriendsBetweenAges(user.age - 2, user.age + 2)
else:
do findFriendsBetweenAges(18, 99)

The problem isn't the implementation, it's the concept in the first place. If you're spinning up a new Lawful Good social media site, you can make it do whatever you want. The problem is how many different states have different laws about how minors can or cannot enter relationships with people of different ages. Then once you stop considering different states, now consider different countries. The internet is accessible to anyone, no matter their age, no matter their location. Writing a website that can handle every possible situation is not impossible, but may be prohibitively expensive.

And of course, I didn't even talk about people lying about their age on their accounts! How does a website even verify that? To what degree are they liable? You want to upload an ID to make an account?