There are absolutely gradations of maturity. That is why we use phrases like more mature, less mature, immature, etc.
Typically, maturity is described in stages. For example, when describing physical maturity we use the Tanner Scale.
There are also stages of psychological maturation as well. For example, Erikson's stages of psychosocial development.
There are many, many other examples of staged gradations of maturity. So, if you wish something more complex than more, less, fully, etc., you will need to delve into the specific field you wish to describe its stages.