Somewhere I have a shot of me standing on the ramp next to a smiling PSA B727 @ SJC just before my first ever plane ride. It was in 1970 from SJC to LAX. My first small plane ride came a year later at SQL. That ride was in an Ercoupe (a little 2 seater with a twin tail and no rudder pedals) I have photos of that flight as well.
For my daughter I have photos from here first ever plane ride when she was only 4 months old. Frontier B737-200 (N214AU) from SFO to DEN.
Although this was far from being her first ride, here is a shot from last week take at ORD just before departure to SJC: