• 年齢ねんれい — age; years of age
• 高齢こうれい — advanced age; elderly
• 年齢層ねんれいそう — age group; age bracket
• 学齢がくれい — school age
• 高齢化こうれいか — population aging
• 高齢者こうれいしゃ — elderly person; senior citizen

The radical 歯は points to teeth, which wear down as we grow older. In 齢, it helps express the passage of years — literally the age someone has reached.