He and Star were both 14 at the start of the series, soon after the start of the school year.
The new school year began, or was close to beginning, in "Sophomore Slump" when Marco leaves for Mewni.
Marco's birthday is mentioned in "Lava Lake Beach" a few episodes later. There's no indication that an ENTIRE YEAR passed in the span of those few episodes.
Star's birthday, Stump Day, is coming up. It's her 15th birthday, we know this from concept art from that episode that was released in the "Stump Day" edition of the guidebook, showing banners that read "Happy quinceañera, Star!" Logically it should take place before the start of the school year, as we saw in the first episode with Star's wand ceremony after her 14th birthday, but as a Holiday Special it can easily be waved off as being out-of-order, like Hungry Larry.
For all its continuity, the show has never been strict about its timeline.
November 28th is Adam McArthur's birthday, he gave that answer as Marco's birthday as an improv line during a Marco LiveChat, and told us Star's birthday was "Stump Day" at the same time, the actually canon and important information. If the November 28th seems to conflict with the canon events depicted in the show, it's not canon.