You chose a complicated abstract topic, and written about it in an academic way.
I would not have introduced the new word 'gladness' - happiness is the key word in the question and it's difficult to paraphrase. 'Gladness' certainly doesn't cover it. You wrote:
It is true that people have different views on happiness. I think it comes from different characteristics and thoughts that people have about the meaning of gladness.
Your introduction is made clearer by your conclusion, where you have some good ideas. How could you rewrite the intro to make it stronger?
People have different views on happiness. I think this difference comes from each person's characteristics, attitudes, and beliefs. In this essay, I will give some examples of why happiness means different things to different people.
'As a matter of fact' seems strange here. It is usually used to give more information about something, but here there is nothing before it.
people with different ages have various objectives and interests ==> people of different ages have various objectives and interests
I would rewrite Paragraph 2 to change the markers:
People of different ages have their own objectives and interests. For example, children are pleased when their parents buy them toys. For young adults, happiness may mean following the latest clothes and hairstyles. For older people, having an admirable career and family may bring true contentment.
In Paragraph 3, you discuss 'instinct,' but is this a chicken and egg situation? Does money make rich people happy, or does luxury lead to more luxury?
In Paragraph 4, you discuss 'access' but it's not entirely clear what you mean. For example, is there a quicker way for people who are interested in education to achieve happiness? Why go through all the pain of studying if there is a quicker way? You also need to give an example for this point:
This goes same for other people with different objectives.
Make sure you develop all your ideas fully. Check the IELTS descriptors for Task 2 under Task Response
In conclusion, considering the fact that individual’s temperament varies from person to person, therefore, the meaning of being happy is quite relied on personality.
'Considering' and 'therefore' can't be used together. They are like 'Although' and 'but.'
In conclusion, because temperament varies from person to person, happiness depends on personality.
Being too ambitious also may lead to disappointing, preventing individuals to attain a happy life.
Being too ambitious also may lead to disappointment, and prevent you from living a happy life.
As I said, you've approached the topic in a quite academic way. Would more examples have made it easier for you to write and for the reader to read?