Here's Peter Kavinsky as described in the To All the Boys I've Loved Before books:
He's described as being tall, athletic, and a "cardboard cutout of a 'cool guy' in a movie about high school." Lara described his look as that of a "Handsome Boy from a different time...handsome enough for a girl to wait years for him to come back from war." He's in the same year of high school as Lara Jean, so he's also 16 or 17 years old.