Kids: The life-energy at its best

Childish, naive, ignorant, student, learner are few synonyms a kid's world is usually associated with. When I tried to have a closer look at the Kid's world, I find exactly the reverse. No KIDDING.
1. Energetic: Try to play with a kid for couple of hours and you would realize how much energy they have. It is endless pool of curiosity and readiness to play, to know things. No wonder toddlers feel bad when it is time to sleep, because they want to keep playing and doing things. Everything interests them, from listening stories, doing household chores, fixing toys, walking/playing in the part/woods, etc. No wonder, fitness experts say that it is better to play with kids than going to a gym.
2. Freshness: Kids mean freshness. They live in the present. One learned saint said that this is the best thing we can learn from kids. Live life in the present and sense the freshness of life first-hand. Kid's life has no place for grudges, sadness, animosity, etc. Kids do cry or get angry/grumpy at times but the next moment everything starts afresh, with new energy as ever.
3. No cheating: There is no malice in a kid's mind. Momentary actions erupting from curiosity might hurt others but these actions are not due to animosity from inside their hearts. As kids live in present, everything starts new and is as enchanting as anything ever before. They have no motivation to cheat.
4. No materialism: By nature, a kid doesn’t accumulate worldly things. They don't have huge demands or expectations from life. Small things make them happy and overjoyed. Small happy world but full of adventures. They don’t need expensive cars, houses, gadgets. Nature and living being is their biggest toy.
5. Love to animals: Every kid loves birds, animals. They want to be with them, take care of them. The most loved cartoon characters are based on animals.

So, when does a kid starts changing and becomes a modern man or women? When does a kids stops living in the present, loses energy, starts eating those animals which (s)he loved in childhood, stops smelling and exuding freshness and starts running after materialism and future?
It is not a question to be answered by science, we need to look inside ourselves to define it and try to answer it.