old age; advancing years
Often used in expressions like 年波には勝てない (you can't beat old age) or 年波を感じる (to feel one's age). The 'oncoming age' gloss reflects the nuance of years advancing like waves.
Lately, climbing stairs is tough; I feel my age.
I can't beat old age; I can't stay up late like I used to.
Compound of 年 (toshi, 'year, age') and 波 (nami, 'wave'), literally 'age waves'. The metaphor of years coming like waves is a natural image; the exact historical origin is uncertain.