Immaturity is not basically connected with ages. You can be serious and mature in 20, and foolish in 60. It doesn't make difference how old you look, what kind of job you have or which is your position in society. Remember Silvio Berlusconi and his Lolita fetish girls. He is enough old to be a proud grandfather, but his behavior would fit better to a teenager.
Life is divided into stages. Childhood, youth, middle age, old age, every period has lessons to learn. If you pass this lessons on time, you are on next stair. You will bring your experience with you and share with your descendants.
What happens when you did not pass life lessons on time?
Which are consequences?
You may act naive. Everyone can make fool of you. You did not kiss a girl until 25, so you don't know how.
Everyone can make fool of you. You are an excellent target for cheaters, impostors, liars.
People will mock you, that you are a greenhorn. You are still on mamma's milk.
You can deliberately hurt someone. You were virgin until 30, now one girl is not enough and you go further.
There are various reasons why you did not participate in an important stage of your life. Maybe parents forbid you to make hands dirty in the sand until other kids played on the playground. Maybe you had a harsh mother who did not let you wear a short skirt and you had limitation to go home before midnight. Maybe you follow your tradition or religious beliefs so you did not pass what you wished. Whatever is your reason, life lessons are cruel. Sooner or later, you will want to compensate what you missed. Then you will be stuck in the trap because you don't know how to deal with this.
Lionel is an older man, he is over 50. He never went into a nightclub, and one day he joined to his nephew. He had an awesome idea, to dance with young girls. You can guess that everyone laughed when he started to jump on "Wiggle Wiggle" song. In the end, he was so pushy, that bodyguards kicked him out of the club.
The other example is Andrea. She is over 55 and she works in civil service. Her colleagues are shocked by her clothing. She looks like a cheerleader in retirement. Short skirt, golden shoes, and artificial blonde hair are bad widgets on her wrinkle face. Her secret came from early days when she was the teenager. Her mother forbids her to wear sexy clothes, because she was afraid that her daughter could be labeled, as the easy girl. Andrea is married a long time, but her awful style is still here.
Greenhorns are people without experiences, eternal beginners. They will start new things and it will be funny to watch. Life circus made them as clowns, to make others laugh.