Labour laws will apply, they probably can't just pick who they lay off. Maybe they have to let go the same proportion of people at all levels of seniority.
Age groups in this case are 10-year buckets, 15-25, 25-35 etc. So if a role has for example 40% of the workers in 25-35, 20% in 35-45, 20% in 45-55 and 20% in 55+ then after the reduction it still has to be as close as possible to that distribution. So it's not like all of the younger workers that are "last in" in their early 20s would be fired, it would be similar numbers across each age group, within each group it's the last hired.