1The 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.
2Denotes 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.
3An old way of telling time under the pre-modern variable-hour system; nanatsu referred to around 4 p.m. today. This usage is essentially obsolete now, appearing only in historical dramas or classical literature.