Spider-Man Far From Home takes place right after Endgame in 2023, but Peter was 17 years old at the time of snap. So, he was 17 at the events of this movie. EDIT: I AM SORRY!
October of 2023 – Peter returns at age 16 still, just missing his August birthday where he would’ve turned 17 years old. Summer of 2024 – Far From Home – Peter is still 16 years old. He will turn 17 years old on August 10th 2024.
However, since Thanos’ blip occurred just before Peter turned 17, and he returned to his current age when he was brought back, this puts his No Way Home age at 17. There is also a chance that Peter turned 18 by the end of the film, as we see him swinging around New York during winter, meaning his birthday would have passed.
In fact, it all seems to have been a blip for them. Since Peter Parker was a 15-year-old sophomore in Spider-Man: Homecoming, and Infinity War took place a year later, he would be a 16-year-old junior in high school — it’s safe to assume, then, that the jaunt to Europe with his classmates would take place between his junior and senior year.