I know many socially adept people who have trouble making friends in their 30s. People in their 30s often are married or in a serious relationship, and spend most of their free time with their significant other. Or, people in their 30s often have kids and have little time to socialize. Also, people at this age may have an established social circle and may put in less effort to build new friendships.
Sure, you can still make friends, but it's definitely a much smaller pool than in your teens and twenties.
I don't know, being in that demo, seems like the main issue is that I don't meet many people through normal activities. And some of the hobby-like activities where I go to meetups are things aren't really filed with people I'd like to be friends with as a "couple" with my wife.
Probably widely varies by profession / lifestyle though.
Sure, you can still make friends, but it's definitely a much smaller pool than in your teens and twenties.