How old is old enough to let my son go next door to play on his own?

My son is 4.5 and loves to play with this kid that lives 2 houses down th street. We live in a quiet neighborhood.My son has high functioning Autism but I'm not sure how relevant it is (it just makes me nervous more than anything cause ya know, he's my kid!)Anyway, thanks via /r/Parenting https://ift.tt/2JaRSZ5