We can all remember ourselves as children. Most of us think that we are still essentially the same as we were then. This is part of a basic belief that we all stay pretty much the same person all through life.
Not who you think you are: different beliefs
There are other beliefs which disagree with this view. Buddhists in particular, believe that the "personality is an illusion." The Buddhist philosophy believes that we all change constantly, and that our belief that things stay the same is wrong.
It is well known that our body replaces all of its cells many many times in our life time. Some cells are replaced in a matter of days, some cells survive for years before finally being replaced. Even if you stay the same weight and height, there is a constant turnover of the physical matter that makes you up.
Science is now beginning to discover that is not just your body that changes. Your mind, your thoughts, your ideas, your beliefs, – they all change too. How you think can be changed by meditation. The basic functioning of your own unconscious mind can also be changed.
A recent study compared the personality of a set of schoolchildren measured in 1950 with those same schoolchildren 63 years later. The study found no correlation between how teachers assessed the children's personality at the time, and how close friends assess their personality now. It appears that every one of these children had changed their personality over their lifetime. Other studies have suggested that for shorter periods, tens of years, your personality stays pretty constant. But this study shows exactly the opposite.
I'm sure that we are all familiar with meeting an old school friend after many years, and being amazed at how much they had changed. Well, it seems that the same applies to you.
Harris, et al. Personality Stability From Age 14 to Age 77 Years. Psychology and Aging. 2016 Dec; 31(8): 862–874. doi: 10.1037/pag0000133