Kirk became a commander in his late 20s. Spock was a commander by 37. In TWOK, when Sulu, Uhura, and Chekov were all commanders, they were 48, 46, and 40 respectively, and we don't know how long they'd been commanders at that point. (Sulu and Uhura were both lieutenant commanders at least 12 years before then, as of TMP.)
As of 2380, Commander Beverly Crusher is 56, Commander Geordi LaForge is 45, and Commander Worf is 40. I assume Commander Miranda Kadohata is around 40 or so, given how long she's been a part of the Enterprise
Otherwise, Captain Jean-Luc Picard is 75, Lieutenant Jasminder Choudhury is somewhere around 40, Lieutenant Dina Elfiki is probably in her late 20s, and Lieutenant j.g. T'Ryssa Chen is 26. Lieutenant Joanna Faur's age is unknown to me. So what you said before is wrong; there are only four commanders in the senior staff, and four lieutenants. Odd absence of lieutenant commanders, though.