Even people in their 30s can tell us.

Theek se brush nahi karoge toh cavities ho jaega, then a 20 year old dentist would scold you.

Chasing after love. Love fades. Start a family young and enjoy time running after your kids.

nothing, just live your life yourself

don't rely on others to tell you how to live it

Nothing much. There are ups & downs that come in life at all ages, and you adapt & deal with changes day by day as things change. That’s life at all ages.

In your 40s, you've gathered 20+ years of experience & wisdom of dealing with adult life & daily changes, so life gets easier in many ways, and many things that would have stressed you in your 20s, don't seem like a big deal in 40s to deal with.

Get married early

Never Drink, Smoke or Chew tobacco.

Not a 40 yr old in my late 20s build muscle since later when u are the muscle is hard to build Make a system for upskilling, passive income since coming times things will be very volatile.
Know this that whatever you do is replaceable so make sure to develop a niche skill that is hard to replicate or replace. Relationships and friendships keep those which u find long term , growth and love. Dump those which are toxic it is very hard to do so but you have to take such a decision. Learn abacus 🧮 it makes your mind fast and improves your cognitive skills. Play or do activities that increase the grey matter of your Brain. Understand that nothing in life is permanent neither victory, defeats or your adversities learn to make peace with this fact so that there is no attachment
Whether you become a billionaire or stay a peasant please enjoy the beauty of life.

Junk food, sugar and cold drinks intake

does junk food really have the impact that people say it does or is it all magnified?

Just 3 things. 1) Don't be too busy with your current job . Develop other hobbies, alternate career paths and travel as much as you wish. 2) Don't be afraid to take risks be it investment, learning or switching job/countries or careers. 3) Take care of your health and also invest effort in finding a life partner when you are young.

As you age you will realise that for an average corporate Joe, it is your family and good health is what gives you more happiness in life than your job and money.