in s1e9, crystal venom, allura was shown during her “bratty teen” phase, with her father, alfor.
an important thing to note: alfor looks in his mid thirties/fourties here, and allura looks around 13-15.
fast forward (in time) to the pilot, in allura’s flashback, right before alfor put her and coran to sleep. alfor looks much older here, showing a major passage of time, while allura looks like the current allura we all know. this means she’s probably in her twenties here. so unless the staff completely forgot about writing crystal venom, allura can’t be a teenager.