Most of the sources that I've come across claim that menarche is considered delayed if it hasn't occurred by age 15 (and whatever equivalent milestone there is for boys, the rule seems to be that the average age for that milestone will be one year later than the female average), around 98% of girls have started menstruating by 15 (or at least girls in developed countries where malnutrition is less likely to be an issue. The body won't prepare for pregnancy if it doesn't have enough fat to sustain one so if girls over 15 haven't had their first period for that reason I don't think that's age-related per se). Some sources claim 16 but 15 seems more plausible to me because menarche is not 'supposed' to take more than 2 years after thelarche to occur and virtually every source I've come across claims that thelarche is delayed if it hasn't occurred by 13 (the exceptions being one AI-generated answer that claimed it's delayed if it hasn't occurred by 12 and a site claiming that the average age for puberty is 13 for girls and 14 for boys which is demonstrably false, it's well-documented that the average age for thelarche is 10 and for menarche it's 12 or at least that's the general or traditional estimate, in the developed world some stats go lower, at least for certain demographics, but I haven't seen anything higher). Some sources will claim menarche is delayed if it hasn't occurred by 15 or within three years after thelarche.
Is or should Tanner stage 5 be considered delayed if it hasn't occurred by 15 (for girls, 16 for boys)? I know that Tanner stages 2-4 (since 1 and 5 are just pre-puberty and the completion of puberty respectively) don't necessarily take a neat one-year to complete but typically menarche occurs in stage 4, yet I've come across sources that claim Tanner stage 5 is delayed if it hasn't occurred by 15 (in girls). Should it be/is it generally considered to be delayed at 16 for girls and 17 for boys?
Are the Tanner stages arbitrary to begin with? It's interesting to note that, apparently, in Tanner's original research some girls who had reached stage 5 reverted back to stage 4 (or perhaps certain characteristics) and many people never reach stage 5 to begin with.