My child prefers to interact only with younger children or adults.

Preferring younger children or adults is often simply because the pace and expectations of the play match your child's developmental stage and feel safe—which is natural in itself. There's no need to force same-age interaction. At the same time, gradually create chances to be with peers; small, short activities built around a shared interest are easier to join. When playing with younger children, keep an eye on differences in power or who leads, so both can enjoy it safely.