I believe these values to be more accurate, however there seems to be mixed information on the minimum level for Temple of Atal'Hakkar, so I'm unsure on that.

Proposed Changes
Ragefire Chasm: 10 -> 8
Shadowfang Keep: 11 -> 14
Razorfen Kraul: 25 -> 19
Scarlet Monastery: 20 -> 21
Razorfen Downs: 35 -> 25
Zul'Furrak: 39 -> 35
Maraudon: 25 -> 30
Temple of Atal'Hakkar: 45 -> 39? 35?
Blackrock Depths: 42 -> 40
Lower + Upper Blackrock Spire: 48 -> 45
Stratholme: 37 -> 45

Sources
https://progameguides.com/wow/wow-classic-dungeon-levels/
https://vanilla-wow-archive.fandom.com/wiki/Vanilla_WoW_Wiki

I can confirm that lvl 28 can't enter Maraudon.