Lexa's around 22ish so they're only a year or so apart, that would work! Maybe they were early on in the romance, Glimmer was helping them out with some stuff, and then Glimmer got into the quarry which gave Lexa major PTSD flashbacks and she was so relieved she lived when Costia died, and has been like ... intentionally keeping her distance because she's afraid she almost got her killed and could ruin her.