One hypothesis of the MBTI is that a relatively psychologically healthy person develops each of the four conscious cognitive functions in roughly each of the first four to five decades of life, so that by age 50, all four functions can be used with ease as needed, though the the first two are still preferred.
For the INFJ, our first four functions are:
1) Introverted Intuition (Ni)
2) Extraverted Feeling (Fe)
3) Introverted Thinking (Ti)
4) Extraverted Sensing (Se)
This process of development has largely played itself out in my life.
I was very imaginative in my childhood (despite a majorly dysfunctional family) - loved art, creative writing, reading fiction, and was very much into the world of make believe (fairies, princesses, and the like). I wasn't as daydreamy as I imagine some INFJs to be - maybe partly because of all that I was exposed to at a young age (hippie parents, liberal public schools/daycare, etc.).
My teen years were a highly relational time for me so that fits with developing extraverted feeling. I hated high school, but I loved my small church youth despite its problems. College was the best - I was very involved with campus ministry. It was in that setting (Intervarsity Christian Fellowship leadership team) that I was first given a version of the MBTI - it was actually the Keirsey Temperament Sorter. I tested XNFJ, but when I read the descriptions, I knew I was INFJ.
Overlapping with college and into my young adulthood, I definitely developed introverted thinking - my analytical skills showed themselves when I questioned my faith and turned to books like Mere Christianity to discover a logical foundation for it. It's also when I was in grad school for journalism at Berkeley where I did my masters thesis on pro-life progressives. And to get into that school, I had to take the GRE - my highest test score ever was in the analytical section (650/800). I also spent a lot of my 20s studying the MBTI, though I didn't become a certified practitioner until my mid-30s - that's a funny story that I will save for another rainy day - it's raining here :)
Speaking of my 30s, it was my season of raising children, so that totally forced me to develop my extraverted sensing since everything was so hands-on!
And now that my kids are all in the double digits (as of a few months ago), that same inferior function is still being developed but in a different way - more in terms of appreciation of the natural world God created. I can't get enough of being in creation, savoring beauty, learning how to be present, enjoying the simple pleasures...and on and on. I love being in my 40s! (I'm 44)
My first two functions are still the ones I use the most, but I am better with the second two, and I hopeful that I will experience more growth with them by the time I am about 50.