Everyone possesses a Persona, but only those who have summoned it before and know of its existence may call it out by will by concentrating one's mind. Personas that have consumed other Personas will be more unstable and will need
Persona suppressors to keep their Personas under control. Although
Persona -trinity soul- is supposed to take place in relation to
Persona 3, Evokers are never used and summoning Personas is an easier task than it was in the game on which this anime was based.
The anime also states that a person's ability to summon a Persona weakens as they reach adulthood, and they may need to take medicine to keep up. This, however, contradicts with
Megami Ibunroku Persona and
Persona 2 , as the rest of the adult cast can summon their Personas without issue, with
Persona 5 Strikers having an
adult awaken to a
Persona as well.
One character speculates that this may simply be due to adults' tendency to lose their strong sense of identity as they grow older, and that it's not inherently about age, but personality. Despite that, however, this is one of the reasons that
Persona -trinity soul- is not considered canonical to the series.