It’s pretty obvious by the title itself- although Taylor does mention her age (19) in the song, which is when she dated him

At some point in their relationship, it's speculated that they were sat closely at an award show, and they were not on speaking terms

Many of the lyrics refer to more mature features of a man, and John was 32 while dating Taylor at 19...as well as lyrics hinting at constant disapproval of others around them

tolerate it describes a power stance relationship and Taylor feeling trapped and unseen/unheard, like a kid. This subject is similar to subjects she’s talked about that are also about John; like “If I was a child did it matter?” from Would’ve Could’ve Should’ve

A lyric parallel is present that is directly related to Dear John from Speak Now which is also about John Mayer. “I damn sure never would’ve danced with the devil at 19” (WCS) and “Don’t you think 19’s too young” (Dear John)