As a matter of fact B is older than U only by a few months and S is older than A only by a few weeks (Check the episode in which they sang their Birth year hits, SR had also performed in that episode). U and B were born in the same year while both S and A were also born on the same year (one year later of B/U).
Agreed that UA were equally good as BS, but this jumping of "hum sabse chhote hai, pls pls pls pls humein vote karo" has to be considered as hyping only.