That wouldn't be a realignment.
Considering Biden won the 18-29 age group by a more than 2 to 1 ratio in 2020, it certainly could be. When I talk about a "re-alignment", I'm mainly referring to a substantial number of demographic groups witnessing significantly larger shifts over the course of a single election than the historical norm would suggest. In this era of hyper-partisanship and extreme polarization, that means it doesn't take drastic, thirty-point net swings or whatnot in order for an election to be classified as a "re-alignment" according to this definition.