What are the best healthy lifestyle habits one should cultivate from very early age and throughout one's life in order to reach old age (e.g. 70s/80s) with optimal health?
Well, this is a big question to tackle considering there are so many different strategies that people take. I'll address the question of maintaining 'optimal health to old age' according to the biopsychosocial model of health.
1. Biology - One of the best habits to develop from an early age is to have a healthy diet and exercise regularly. A healthy diet includes eating more vegetables, cutting back on the sugary beverages and fried food (trans fat). For exercise, the American Heart Association suggests "at least 150 minutes per week of moderate exercise [~20 min./day] or 75 minutes per week of vigorous exercise (or a combination of moderate and vigorous activity)." This habit is important for maintaining a healthy BMI and reducing the risk for many chronic diseases such as diabetes, cancer, etc.
2. Psychology - A critical skill to develop is mindfulness - being able to anchor one's attention to the present experience. This skill includes learning to handle stress, process one's emotional state, overcoming self-destructive thoughts, understanding who you are, etc. All are important to maintaining good mental health. Mindfulness can be developed in many ways: writing, meditating, paying more attention to the world around you.
3. Sociology - The main habit to cultivate here is to build healthy relationships - with family, friends, workers, etc. It's important to have a support network during times of trouble. Not to mention the amount of stress (and resulting illnesses) that can be avoided by giving more care to the social part of life. Personally, I've found the best ways to learn about building healthy relationships is through: listening, reading, empathy.
My answer is of course not exhaustive - I merely gave a good faith effort at pinning some of the more general habits that have long term health effects. For example, another simple habit under the biology category is to avoid tobacco use.
Finally, I do want to point out that a habit that contains all of these best lifestyle habits is: take an interest in your health (a.k.a. learning!). Don't stop at just reading this answer. Keep an eye out for the news on the latest health research, public health promotions, articles on the net (do check the validity), etc.