I'd say 50+ when the only mobs that give exp, hit points go way up.
It's just not as efficient to melee as a shaman in the 50s. Doable though.
I've never gotten the shaman epic but I read that it is a game changer. I advise you to save up for that if you want to main the shaman.
You can even melee very well with it if you want to.
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Spectres in Feerott Till 44 Then Blood Gills in LoIO till 49-50. ez-pz, I leveled a shaman (twinked) to 50 in less then 3 weeks solo.
I did level 45 in 10 hours flat at bloodgills.