Here is a 27-year-old version of the megastar glowing at the "Selena" Hollywood Premiere back in 1997 (left). This was J Lo's breakout role and shortly before her debut her debut album, "On The 6" ... which featured bangers like "If You Had My Love" and "Waiting For Tonight."

And, nearly 3 decades later ... and just two months shy of her 55th birthday ... Jenny From The Block was all glam earlier this week, looking ageless at The Egyptian Theatre in Hollywood for her new movie, "ATLAS."

Yes, she'll get you "On The Floor" ... but the question here is: