Under

We use under, not below, to refer to age:

You have to be under 18 to get an allowance.

They have three children under the age of five.

We use under, not below, to talk about measurements of time and weight:

We finished the project in under a year and a half.

The bag was just under 10 kilos, so I was able to bring it on the plane.

When we talk about height and temperature, we use below not under:

The roof of the new building is just below the height of the church and I think it distracts from the church.

Not: … under the height of the church …

The liquid must be kept below five degrees. (preferred to … under five degrees.)

Underneath is similar to under, but it usually only refers to position:

Underneath the stairs is where we keep our vacuum cleaner and brushes.

The child weighed under five kilos.

Not: … underneath five kilos.