noun
lowering of age; reduction in age
Refers to a trend where the average age of a group (e.g., participants, users, victims) becomes lower. Often used in social or statistical contexts.
スマートフォンの普及で、インターネット利用者の低年齢化が進んでいる。
With the spread of smartphones, the age of internet users is getting lower.
犯罪の低年齢化が社会問題になっている。
The lowering of the age of criminals has become a social issue.
高齢化 means 'aging' (increase in the proportion of elderly people), while 低年齢化 means 'lowering of age' (decrease in the average age of a group).
Compound of 低 (low) + 年齢 (age) + 化 (-ization). A straightforward modern coinage.