Of course, I mean besides Lt. JG Picard in "Tapestry":
Is it just Hollywood bias or are we to believe that officers either get promoted, get killed, or retire if their career is not advancing?
Lt Jg Reg Barclay was (approx) 42 years of age when we meet him in TNG: Hollow Pursuits. That seems rather old for such a low rank.
Lt Jg Janice Rand was (approx) 51 years of age when she was promoted to her rank.
Ensign Alans was a technical specialist (a volcanologist).
When we meet him, he's (approx) 45 years of age.
Ensign Cheney's age isn't described in canon, but she doesn't look that young (mid-50's?)
Obviously it's a bit different for longer-lived species, but Ensign Chu'Lak was aged 77 when he was commissioned an Ensign.
And Ensign Tuvok was approx 85 years old when he rejoined Starfleet as an Ensign.