On August 15th 2017 we increased the maximum level for badges to level 100. When a badge changes from a non-leveled to a leveled badge, or there is an increase in its maximum level, the badge level is retroactive to the date of the original badge creation. You can find the badge creation date in the badge detail page.
This is the default setting for Untappd for all badges; the badge's level is always retroactive to the date of its creation. This means if a badge was created on 4/1/16 and turns into a leveled badge (or increases to 100), your level is retroactive to the date of its creation.
This is not to be confused with Retroactive Badges - if you enabled the retroactive badges setting, it will look at your entire beer history, not just the history between your most recent check-in and the badge's creation date.