Kylie Jenner And Tyga 18 Makes Relationship Fair Game On Reality Show

Kylie Jenner and Tyga are now fair game for producers of "Keeping Up with the Kardashians" ... and it took no time at all for the cameras to capture their relationship.

The show didn't really touch on the relationship, because Kylie was 17 last season. But one birthday makes a big difference, so 'KUWTK' cameras were all rolling Tuesday during the family vacation in St. Barths.

Tyga and Kylie weren't flaunting it before ... but now they're selling it.