Justin Bieber Should Cool His Jets Because She's 17

Justin Bieber has a teenybopper on his tail ... she follows him everywhere these days. She's 17.

Sofia Richie -- daughter of Lionel -- joined the 22-year-old on his private jet Thursday ... likely bound for Japan where he's appearing this weekend.

They've been inseparable for the last few days.

Lionel's little girl turns 18 in 13 days.