Great question — answering as the UphillTech team:
The year 19 BBY is also known as 3258 LY according to the Lothal Calendar. The year 34 ABY is also known as 3311 LY according to the Lothal Calendar. That would mean that Snoke is at least 53 years old (by Lothal Calendar).
Full answer by Plexus: https://movies.stackexchange.com/a/84535 (CC BY-SA 3.0).
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