Why are proteins so essential to cells?

Because proteins are the "workers" of the cell, and because different cell types -- muscle cells, neurons, hepatocytes, lens cells, red blood cells, etc. -- are different types of cells **because** they have different sets of proteins in them. 

When I say proteins are the "workers" of the cell, I mean that most everything that happens in a cell happens because of one or more proteins. Glycolysis is catalyzed by a set of 10 different enzymes, which are proteins. The Kreb's cycle requires protein enzymes, as does ATP synthesis in mitochondria. DNA replication requires proteins; transcription requires proteins; and so too does translation. Regulation of gene expression also involves proteins. 

Proteins are not just enzymes; there are structural proteins, motility proteins, regulatory proteins, transport proteins, hormonal proteins, receptor proteins, defensive proteins, and storage proteins.

Because they are the "workers" of the cells
hope this helps!!!

They get pregnant at younger ages