Definitely not constant for me.
For the first 17 years, I grew up as an ISFJ. I was shy, self-conscious about my everything. My low self-esteem was a disaster. I could never imagine I would be able to open my mouth and get attention in any conversations. I did everything on everyone else's behalf but mine. Trying to say "No" gave me a hard time.
In the 18th year, I was blahh... mediocre, i would say... I don't even know what type I was at the time.... I was changing but wasnt aware of it.
The first day I stepped into college, I became an ESFP. Odd huh?! I was surprised myself. Almost everyone at the orientation knew who I was. I became pretty
well-known during my first year and 1st half of 2nd year. I got excited and animated every time I was around people. Making ppl laugh made me feel recharged. I felt free, complete, and at ease with myself. Confident would be a good word to describe me at the time.
As the 2nd school year started, I met my second closest friend. There was something very unique about her that really attracted me. The way she gave people (including me) advises just opened my eyes. She was 'unconventional'. She came up with thoughts that most people didn't think of. One day I just asked her to do the Jung personality test for fun. It came out she was an ENFJ. I then found the most suitable word in the description of ENFJ to describe her was
charismatic.
From that day, I started to read into a lot of books, articles, websites on personality types. I wondered how 4 letter words made such difference about each individual. Reading Keirsey's Please Understand Me just made me idolize NF at the time

I wished I could become an N

But I knew I couldn't. In most of the conversations with my friend (the ENFJ), I always felt I couldnt go beyond my limit. At certain points, it got too abstract for me. I always got irritated and wondered why she just couldn't see what I saw, instead of thinking of something so "unrealistic".
Then on a beautiful winter day, I realized I had become an N. Please don't ask me why. I changed before I noticed because I never thought it would be possible (from an S dominant to an N dominant and vice versa). But for the first time, I knew what "head in the clouds" meant. For the first time, all the books I've ever read started to interconnect with each other in a flash of thought. For the first time I thought that I could write (even though my 18 years of being an S didn't provide me much vocabulary or writing skills; and besides English was not my first language) For the first time I could had a real conversation and felt connected with my INFJ brother who, before, I constantly had a fight with due to my criticizing him of living in the clouds and neglecting daily matters (finance and such). For the first time, I could say I saw "
BIG"
Anyway I could really go on and on with that

I guess you got the idea.
I read into neurology lately and I think it's completely possible for us to change our type.
p.s. I realized I used too many
I's. Sorry for wearing you out (if you read).