The counter for seven items (nanatsu) — part of the traditional native Japanese counting system (hitotsu, futatsu, ...).
ケーキを七つに切ってください。
Please cut the cake into seven pieces.
箱の中にはリンゴが七つ入っている。
There are seven apples in the box.
子どもが七つの星を描いた。
The child drew seven stars.
Denotes age seven — an older/native way to say 'seven years old' (still used in spoken Japanese).
娘は今日で七つになった。
My daughter turned seven today.
彼はまだ七つで、小学校に入ったばかりだ。
He is only seven and has just started elementary school.
In older time-keeping systems, 'nanatsu' sometimes indicated the fourth hour (around 4 p.m.). This usage is archaic and rare today.
古い日記には「七つに出発した」と書かれており、この場合は午後四時を指す。
An old diary says 'we departed at nanatsu', and in that case it refers to around 4 p.m.