Maybe it is because of age, I often think of those old days, though they were all unhappy memories.

The sentence is a little bit awkward but I totally understand what you're saying! You might say instead "Though they were all unhappy memories, I often think of the old days. Maybe this is because of my age."

When I was a teenager, living in our village was really bahard.

"Bad" is not a word in English that can be used to describe other verbs (in this sentence, "bad" is meant to describe the word "living"). Instead, you may say that living was "hard" or "difficult." Or, if you want to keep the word "bad," you might restructure the sentence to be something like "When I was a teenager, our village was a bad place to live."

NThere was no electricity, no tap water, and the ground was naturally soil.

Technically, the sentence you wrote is a sentence fragment. This is totally okay in "journalistic" writing or "informal" writing, but I just wanted to make sure you knew that these types of sentence fragments are grammatically incorrect in formal or official contexts! :)

On rainy days, the ground was very muddy, and we had to wear deep waist rubber shoes to go in or out of the village.

You could also use a semicolon here! It would look like this: "On rainy days, the ground was very muddy; we had to wear deep waist rubber shoes to go in or out of the village." Semicolons are used to connect two separate but connected sentences, which makes the word "and" unnecessary in that case!

When night came, the whole village was very dark because we justonly used the oil lights to illuminate.

"Only" might be a better word here, and "oil lights" instead of "the oil light," assuming that there was more than one light. If there was only one light for the village, you could say "the whole village was very dark because we all shared one oil light."

We just eate sweet potatoes and some other coarse grains.

We usually wore clothes that our older sisters or brothers left.

"Our older sister" makes it seem like you all share one single sister. When you refer to the separate siblings of multiple people, you say "our sisters" or "our brothers" :)

Sometimes, we even wearore cotton jackets without underwear and sleep directly on the reed mat in the winter.

However, lLiving conditions have gotare perfect nowadays, but I still can't forget those hard days.

There are a few ways you could form this sentence depending on what you wanted to emphasize. If you want to keep the word "However," you could say: "Living conditions are much better nowadays. However, I still can't forget those hard days."