It's not limited by character level in the same way weapon skills are. It is limited, but differently.
LVL Minimum Crafting Range now allowed
5: 0 - 75
10(?): 75 - 150
20: 150 - 225
35: 225 - 300
A green iron hauberk, with its trade skill level requirement to craft of 180, is well within the range of crafting for a level 31. I haven't ever seen a craftable that you'd be unable to craft because of level restrictions if you could wear it at the same level.