The word 'young' is hugely subjective. For example, in working class communities, having a baby in your late teens/early 20s is normal. In middle class communities that's young.
To a teenager, a person in their mid-30s (like DH and me) is old. To my parents (in their 60s) it's young.
WRT your nephew - if he's not grown up by age 30 it might be his attitude, if might be the way he was parented, it might be undiagnosed SEN, it might be mental health issues, it might be he's just a fuckwit. Impossible to know.