Tomorrow is the Japanese national holiday called "Respect for the Aged Day" in Japan.

This flows better.

If I hadget it, I would not know who my loved ones are, or where I am...it's so tragic.

In Japan, which is a country with an aging population, it is saiI've heard that more than 15 % of elderly people of my age suffer from dementia.

"It is said" is more formal/stiff. I'd probably hear it in a news article or something.

I had better take aget good sleep, do exercise moderatelyevery now and then, and communicate well with others.

I can't thank my friends on LangcCorrect enough.

If I hadid, I would not know who my loved ones are or where I am...it's so tragic.

or, "If I had it, I would not know..."

In Japan, which is a country with an aging population, it is said that more than 15 % of elderly people ofaround my age suffer from dementia.

I had better take a goodget adequate sleep, do exercise moderately, and communicate well with others.

Not sure how "communicate well with others" helps prevent dementia? So, I'm not sure if you meant something else.

Tomorrow is thea national holiday called "Respect for the Aged Day" in Japan.

I apologize for not having a better explanation, but this is just how I would say it as a native speaker.

If I had it, I would not know who my loved ones are or where I am...i It's so tragic.

In Japan, which is a country with an aging population, it is said that more than 15 % of elderly people of my age suffer from dementia.

"15% of elderly people of my age" can also be written as "15% of elderly people my age."

I had better take aget good sleep, do exercise moderatelymoderate exercise, and communicate well with others.

"Exercise moderately" is not wrong, but usually when we refer to exercise as a habit, we use it as a noun instead of a verb. Therefore, we use an adjective instead of an adverb. Saying "do exercise moderately" slightly implies that you should avoid exercising too vigorously - I don't think that's what you mean.