I agree with Amy's corrections, but with one comment.

If your grandparents are still alive, you would say:

"my grandparents got married when they were 20 years old and they have been together for 60 years".

However, if your grandparents have passed away, it's OK to use the sentence as you wrote it:

"my grandparents got married when they were 20 years old and they had been together for 60 years"

Traditional family

My family is a traditional family. Only one of my aunts is separated, but she never got divorced from her husband. Nobody in my family is a single parent for now and all my cousins, who have children, are married. Only my sister and I live with our boyfriends, but we aren't married. And my family is very big!
At present, neither my brothers nor I live at home with our parents. We left home when we started at University and we've been living away for several years. When I finished my studies, I had to return home because I was unemployed but, luckily, this situation didn't last long because I found a job soon.
Despite these traditional customs, there are many differences between the three generations in my family. For example, my grandparents got married when they were 20 years old and they have been together for 60 years. My grandfather worked while my grandmother ran the house. However, my parents got married when they were 30 years old and both work. And me... I am 30 years old and I don't think I'll get married soon!