Making and maintaining close friendships gets genuinely harder in adulthood, mostly due to reduced natural opportunities for repeated contact that existed automatically in school or early jobs - but it's still very achievable with a bit of deliberate effort.
Regular, smaller contact - a recurring standing plan, a consistent check-in - builds a friendship more reliably than infrequent big gestures separated by long silences.
Adult friendships often stall because everyone is waiting for someone else to reach out first - being willing to consistently be the one who initiates plans meaningfully increases how many friendships actually develop.
A shared regular activity - a class, a hobby group, a recurring meetup - creates natural repeated contact that mirrors the built-in opportunities of earlier life stages, without needing to force something purely social from scratch.