Minor thinking changes can occur as a person gets older, such as being a little slower to bring things to mind or finding it harder to multi-task.  These changes are subtle and do not interfere greatly with daily life.
Significant changes in memory or thinking are not a normal part of aging.  Loss of memory or change in behaviour and/or thinking skills that is severe enough to interfere with daily life should be investigated.
Changes often emerge gradually and it can be hard to know whether to be concerned.  If you are unsure about the changes you have noticed, consider seeing your GP to determine whether further investigation is indicated.