Living things grow and develop
Every living organism begins life as a single cell. Unicellular organisms may stay as one cell but they grow too. Multicellular organisms add more and more cells to form more tissues and organs as they grow.
Growth and development of living organisms are not the same things. Growth is the increase in size and mass of that organism. Development involves transformation of the organism as it goes through the growth process.
Think of a newly born baby. It has all the features of a fully-grown adult, but they are very tiny. As the years go by, they become big and become a young person like you, and later on, into a fully grown adult, maintaining all the features that they are born with. This is growth. But in their mummy’s tummy, they started off as a single cell, and transformed onto a zygote, and into a foetus before transforming into a tiny baby.
In some organisms, growing involves drastic transformation. Think of a butterfly for instance. It starts off as a cell (egg). Then it transforms into a caterpillar, then into a pupa (chrysalis), and then pops out as a beautiful butterfly.
Plants often start from a tiny seed, and grow into a big tree. One thing common to all organisms is that they grow or develop to look just like their parent species, even though there may be some slight variations resulting from the mixing of cells by the parents. (More of genetics here)
Cell growth and development include its repair. As cells grow old, they wear off. Sometimes they suffer injury and bruises, but they are able to repair themselves by growing new cells in a process called Mitosis.
As living things grow, they undergo a process called aging (age). As they get close to the end of their lifespan, their ability to carry out life functions reduce. Eventually, they die to end the process of life.