There are I know cases where Lee's dialogue doesn't match up with what we see the artist doing, but I see no reason to think this is one of them. You can say Ditko's drawn Betty to look older than a teenager, but Peter in that panel doesn't look like a teenager either to me. Ditko's clearly the prime mover behind the Peter / Betty romance and I don't think he'd have intended Betty to be significantly older than Peter. (It has been suggested that the Peter/ Betty relationship was wish fulfilment on Ditko's part, based on his crush on Flo Steinberg, Lee's secretary: Steinberg was twelve years younger than Ditko.)