What are good habits to start young that will significantly make your adult life easier?

And its impact will be significant.
It's not a theory. It's not a feel-good charm. It's not a wishful thinking. It's reality.
The reality looks like that:
So, if you learn at young age to exercise, meditate, keep a journal, cultivate gratitude, wake up early, do the most important task first thing in the morning, write 1,000 words a day, set priorities for your day, pray, smile at people, text a friend, stretch, eat whole foods, speed read, track your spendings, read a book for 30 minutes a day, watch one TED talk a day, study scriptures, learn one new thing every day, do a random act of kindness, floss your teeth, manage your budget, drink water, do one thing out of your comfort zone, sleep long enough, relax, ETC., you will reap significant benefits of those activities as an adult.
There is no way you won't. This is how the world functions.
Habits compounds with time. After a year you reap some benefits, after 10 years you reap benefits not 10, but 100 or more times better.
If your body will be stronger and healthier, if your mind will be more flexible and quick, if your spirit will be more peaceful and joyful, if your relationships will be richer and numerous, if you gain more knowledge and skills, then your life will be easier. It's as simple as that.