岁 (traditional 歲) was once the name of Jupiter, whose 12-year orbit marked the ancient calendar cycle. The simplified form stacks 山 (mountain) over 夕 (evening) — years accumulating like mountains of evenings.
岁 counts age: 你几岁? (how old? — for kids), 我五岁 (I'm five). 岁月 ('years and moons') is poetic time itself. At New Year, 压岁钱 is the red-envelope money that 'presses down the year' — protecting children from a legendary monster named 祟 (suì, same sound). To 长一岁 (grow one year older) happens for everyone at Spring Festival, regardless of birthdays — traditionally, age is collective.
Mountains 山 of evenings 夕 stacked up — YEARS OF AGE.
压岁钱 (New Year money) 'presses down suì' — a pun defeating the monster 祟 with cash. The tradition is at least 1,000 years old.