EXCLUSIVE
The party took place at a restaurant inside the Staples Center -- and you can tell from the pics, these were happier times for Sterling ... possibly happier because he thought he was dating a woman 10 years younger than she actually was.
The cake says 21, but we know from court docs ... Stiviano's actually 31.
Sources close to V. Stiviano tell us, she has notoriously lied about her age for years -- so it's not a stretch to think Sterling believed his sidepiece was barely legal.
Not like Sterling would be able to tell the difference anyway.
