The concept that making friends is easier when you are a kid is not just a popular conception, but actually true. Science has demonstrated that making friends is harder after people turn 30. That being said, it's still important to make and have friends. Humans are social creatures that need a good, full set of relationships, and as people age, new relationships might be necessary to replace family members who have moved away, children who grow up and start their own lives, and even fill gaps left by older individuals who pass away. Having said that, with a little intention and work, it is still possible to make friends as an adult.
The first thing you have to do is deal with yourself first. Check in with yourself emotionally. Putting yourself out there and opening up for new relationships is emotionally challenging, and you might not be in an immediate position to do it. Do you have any personal needs you need to deal with first on your own?
Second, make sure you have the resources to make relationships and social matters happen. Is there enough free time in your schedule? Do you have enough discretionary income to afford doing things? Do you have enough energy to go out and be social?
Third, pick your passions over people. It can be easy at first to just start firing off event or schedule suggestions to people at the fringe of your current social circle, but to find relationships that will carry themselves for the most part, it is better to have relationships with individuals that share passions with you. If you love hiking, identify or invite others you know that also enjoy the activity. Make a list of people that you know that read a lot like you maybe do.
The last step is the simplest, but maybe the hardest, and that's to be proactive. It's not enough to create space in your life for social matters or be open. You have to be the initiator. Call up potential hiking buddies and suggest a trail and particular weekend. Email fellow readers and set up a book club.
Other steps you can take that can lead to more social activity in your life include refreshing your social media use, using your children and/or pets for finding connections, going to night school and volunteering.