Age groups
Up to the VES organisers to decide, UCI is end of year which according to Sossy above MTBA follow.
When a series starts in one calendar year and finishes in the next calendar year - you are in the age group of the last calendar year of the series, not the age group at the end of the calendar years in which the series commenced.
An exception to this, MAY BE (insert uncertainity) that if you only compete in the series events of the first calendar year then you may compete in the age group at the end of that year, and not compete for results in the rest of the series which is in the next calendar year. You can only compete in one age category in a series and have results in one age group in a series.
Now I am as confused as everyone. But it is up to the VES organisers to decide on their rules.