why do mental health problems mostly begin in adolescence?

Although modern mental health is based on the tenets of Behaviorism, from B.F. Skinner, who wrote that "there is nothing within a person that can explain current behavior", both mental health and Skinner are wrong.

At birth, the MIND of a child is fully functional, while its brain is not yet operational. To seek its new identity, the child's MIND vibrationally seeks out who and what the child is from the MINDs of its parents. It uses what I refer to as DEc, or Direct Esoteric communication... or MIND to MIND communication. More enlightened psychotherapists refer to this as "empathic understanding" wherein one uses what is stimulated within ones own MIND to ascertain what is actively being repressed within another's MIND.

What the child's MIND picks up is pain, fear, perhaps even terror in the case of a woman having her first child, and all these intense and negative emotions are misperceived by the child's MIND as either a reflection of who and what the child is... or that they are "caused" by who and what the child is. In either case these cause the child to order his/her MIND to deeply bury these and to never reveal then. The MIND of the child complies, and they constitute what I refer to as the Negative Self-Image, or NS-I within every person.

Ones NS-I impacts the development of ones personality, and along with ones Fate Karma, aspects of ones NS-I are used to create and fix ones personality.

This is the reason that all "mental health problems" are directly tied to the NS-I within a person. But since modern mental health professionals have no idea what the MIND is, or how it works, they are in denial of both the MIND and its repressed NS-I.

Peace

Because except for the first three years of live it's when the body and brain under go the most changes. There for the best chance for something to go wrong. The brain particularly changes more during the ages of 13 and 20 than it does at any other time except during utero and the first two years of life when new pathways are being developed. Unless there's been some kind of brain damage. Than that brain will again undergo some change in order to replace the path ways that were destroyed due to the injury. So the possibilities that some thing happens to change or disrupt that growth is very high during adolecence.

Because its when a person is most vunerable and when they are open to any pain.

They can start whenever. But you are right, most problems begin in adolescence. That is a key part of a persons life.

Because that's the period where the individual is searching for his/her identity.