In the Buffy TV series the episode 'Normal Again' suggests that Buffy Summers is a schizophrenic in a mental hospital and her being the Slayer is simply a hallucination caused by her illness. This means that her delusion starts during the Buffy movie when she first meets Merrick and ends with the last scene of the TV series where she destroys Sunnydale, triumphs over the ultimate evil and is told from now on she must 'live like an ordinary person', Buffy regaining her sanity once more after 7 years.
Another theory is that both Asylum Buffy and Sunnydale Buffy are real and have some sort of psychic link across the dimensions which drives Asylum Buffy crazy. After the end of the TV series Buffy is only one of thousands of Slayers so her calling no longer dominates her life, allowing Asylum Buffy to regain her sanity and for both to live more or less ordinary lives, both still aged only 23.