Yes, you can be friends with someone under 18, and such friendships are normal and healthy across many contexts. Age-based friendships occur naturally in schools, neighborhoods, sports teams, hobby groups, and online communities where people with shared interests connect regardless of exact age.
However, the nature and appropriateness of such friendships depend heavily on context. Friendships between peers of similar ages—such as a 16-year-old and a 17-year-old, or classmates a few years apart—are developmentally typical. Friendships between adults and minors require clear boundaries and appropriate settings, such as mentorship programs, family relationships, or adult-supervised activities like youth sports coaching or academic tutoring.
Red flags that indicate an unhealthy or potentially exploitative dynamic include secrecy, isolation of the younger person from peers and family, inappropriate conversations or requests, gift-giving that creates obligation, and gradual boundary-crossing. Healthy cross-age friendships typically involve transparency, appropriate boundaries, and respect for the younger person's developmental stage.
In professional and community contexts, adults who work with young people—teachers, coaches, counselors—have special responsibilities to maintain professional boundaries even while being friendly and supportive. The key principle is that any friendship involving a minor should prioritize their safety, development, and wellbeing, maintain clear boundaries, and be open to scrutiny by parents, guardians, or other responsible adults.