Where is the nucleus located in an immature and a mature plant cell?

In most plant cells, regardless of their level of maturity, the nucleus is contained inside the cell throughout the cell's life. A few types of plant cells, however, die upon maturity, so they lose their nucleus and other organelles at the time they begin to carry out their real function of providing support for the plant. 

Plant cells can generally be separated into three different types: parenchyma, collenchyma, and sclerenchyma. Parenchyma cells remain alive upon maturity and function in photosynthesis, cellular respiration, and storage for the plant. Collenchyma cells also remain alive upon maturity and help provide support for growing sections of the plant such as the roots, shoots and leaves. My favorite example of collenchyma cells are the 'strings' that run through celery stalks. Sclerenchyma cells are dead at maturity (so they no longer have a functioning nucleus) and are unique because of their thickened secondary cell wall made out of lignin. This helps the cell have the necessary strength to provide support for the plant. Some examples of sclerenchyma cells are sclereids that form structures such as hard seed coats and pea pods, and fibers such as jute and hemp. 

So, many plant cells are alive and have a nucleus as they carry out their function for the plant, but some cells such as sclerenchyma cells die upon maturity, and thus no longer have a nucleus. However, typically, in immature and mature plant cells, the nucleus is located in the same general area inside of the cell.