How to tell age of PSA graded cards, numbering system

[swarmee]

Yes, cards that were graded as cert #20,000,000 were done in approximately mid-2012. Here's a 2012 Bryce Harper with a 20,700,000 Cert.

2012 Topps Pro Debut - [Base] #145 - Bryce Harper [PSA*9*MINT]
Courtesy of COMC.com

Around 23,000,000 they started adding the hologram to the front of flips. Around 28,000,000 they started adding the PSA Lighthouse label.
Higher than 30,000,000, they would need to have the PSA Lighthouse label. Many of PSA's VERY OLD grades had Cert numbers from 30Million, 40M, 90M, so they had to skip those numbers when their numbering system finally caught up to those certs. So if you see any cards between 30M and 43M, they need to have the most current lighthouse label to be graded between 2012 and now. It's still slightly possible they were graded 20 years ago and were sent in for reholdering recently, but if they're from your personal collection, that doesn't really apply because you would know which cards those are.