놀다 takes a preceding -아/어 verb that says how the hanging out happens: 나가 놀다 (go out and play or hang out), 어울려 놀다 (mix with people socially), 모여 놀다 (get together). For adults 놀다 is not childish play — it covers any unstructured time spent enjoying yourself, which is why 나가 놀아라 works as advice to an adult glued to their phone.