harvest, ripen
稔 centers on crops reaching full maturity and being gathered: the process of ripening and the resulting harvest.
稔 combines 禾 (grain plant) with 念, which likely contributes the sound and may suggest the idea of thinking about or tending the harvest. The exact historical development is uncertain.
Grain 禾 on the left, and 念 (thought/care) on the right: picture a farmer carefully watching over the grain as it ripens, ready for harvest.
For ネン, imagine the grain ripening year after year: 年 (ネン, year) -> ネン, and each year brings a new harvest.