Almost everything in a relationship is determined by age. From the age of consent to the level of maturity expected. Men will prefer women of a particular age-set because of their fertility, youth and beauty. Similarly, women will prefer men of a particular age-set because of their protection, provision, and sexual performance capabilities.
Apart from the specifics of men and women, a person's general personality and character is determined and judged as per their age. This is through experiences and knowledge. Consequently, we consider their age when we gauge the wisdom they should exhibit and the judgement we believe they should uphold.
There are things we expect and don't expect from someone because of their age. This in turn determines whether we enter into a relationship with them. The success and failure of someone is also based on their age. Consider a broke sixteen year old and a broke twenty six year old. In fact, it's been said that once you hit twenty six you are poor, not broke.
We all know what we want in relationships. If you are able to get what you want irrespective of age, then good for you. But the idea that age is just a number, and the manner in which we want to disregard it is simply unfortunate.