Generally speaking, it's not an absolute rule but my parents have been outside their native homelands in Africa for about 16 years now. The way they speak English may have changed (in terms of the speed) but their accent sounds exactly the same.
Meanwhile my accent as some say is a hybrid of Irish and American. I should note that I'm only in my early 20s whereas my parents are in their 50s.
Now why is this the case? Do they just not want to change their accent? I think it might have to do with socialization. My parents mostly socialize with other Africans and virtually have no Irish friends.
My parents are friends with an African man who's the same age as them and has spent around 20 years in Ireland but he sounds completely different. But that might be because when he first came here, he shared a house with his white Irish wife and their family. He married her in Tanzania but then moved to Ireland and got a job while living in their parents house. He doesn't sound completely Irish but way different from my parents.