Aging skin becomes less resistant to infection and other diseases because the number of decrease and are less efficient.

There are specified form or types of cells inside the human system, as they serve for the development and sustaining the overall functions of the human body.As there are immune cells like the macrophages, lymphocytes, neutrofils, and many more which are present for the defense mechanism of the body system. While, there are certain limitations for each of these cell's type, like the cells may out run of all there resources or they may degenerate with time causing great distress inside the body system. Aging skin becomes less resistant to infection and other diseases because the number of macrophages or the immune cells decrease and are less efficient.                                                                                                                            

Centriole.centrioles are present in animal cells but are usually absent in plant cells.

No you are not wrong