58-year-old Journey guitarist Neal Schon still ain't too old to crack sex jokes about his 47-year-old GF Michaele Salahi -- and that's exactly what he did during her birthday party last night ... in front of a table filled with people.

The awkward moment was all caught on tape during Michaele's 47th birthday party at a Maine restaurant. In the video, the band strikes up an accordion-filled rendition of Happy Birthday -- after which Michaele asks Neal (who's filming) to blow out the candles with her.

Neal shoots back with the following zinger ... "You blow and I'll watch." He adds, "Watch your hair though it could get gooey."

Lucky for him, the entire table laughed ... including Michaele.

But it's no surprise -- we're told Schon plunked down over $50,000 on Michaele's birthday gift, including a brand new white Porsche Cayenne, jewelry, clothes, and flowers.