Should I change it to the past tense?

Take my family as an example: in the generation of my grandparents, most of these people did not have the chance to access to knowledge. The reason is that at that time the whole society was less affluent; a lot of the families were even too poor to meet their basic living requirements. Therefore, most people during those decades did not receive education.

Should I change the “is” to “was” for the consistency of the past tense?[/u]

It’s still the reason so present tense is acceptable. You can change it if you wish, but I’d leave it as it is.
There are other changes I’d make though:
Take my family as an example: many people of my grandparents’ generation did not have the chance to access knowledge. The reason is that at that time the whole of society was less affluent; a lot of families were too poor to meet their basic living requirements. Therefore, most people during those decades did not receive a good education.

but I’d leave it as it is.<<

I would leave it as it is too. I feel it like more sensible.

kind regards.

I see.Thank you guys.