So how can Peter still only be 16 in this movie? Even ignoring the 5 years he was gone that would mean he was only 13 in Civil War...
He was 15 in Civil War and 16 in both Homecoming and Infinity War, and since he was dead for 5 years, he was resurrected at exactly the same age he was when he died.
Since Far From Home is set in July and his birthday is August 10th, he's on the cusp of turning 17.