Maybe not societal "pressure", but a majority of people certainly think it is abnormal to date outside of your immediate age group. Maybe it's just a reaction to a societal norm.
Back in 1993 when I 26 I dated a 33-year old and a 20-year old in the same summer. The 20-year old was mature, grounded, intelligent, etc. The 33-year old was an immature, jealous basketcase. Physical age and emotional age usually do not match.
It's not about age as much as it's about where they are in their life compared to you. It happens to mostly correspond with age, so that's what we use as a measuring stick.