"The middlescent usually have to bear much responsibility."
Is this sentence correct and natural? I think the word middlescent is not common, which makes this sentence a little unnatural.

I guess a more natural way to phrase it would be:
"People in middle age usually have to bear much responsibility."
or
"Middle-aged people usually have to bear much responsibility."

Am I right?

Question

"The middlescent usually have to bear much responsibility."
Is this sentence correct and natural? I think the word middlescent is not common, which makes this sentence a little unnatural.

I guess a more natural way to phrase it would be:
"People in middle age usually have to bear much responsibility."
or
"Middle-aged people usually have to bear much responsibility."

Am I right?

Answer

"Middlescent" is formed by blending "adolescent" with "middle aged".  It is an appropriate word to use for a middle aged person who behaves in a juvenile manner.  It is NOT a synonym for "middle aged".

So, no, your sentences do not mean the same thing.  You would need to say
"People in middle age who do not act their age should be held accountable"

Answer

I'm over 60 and fairly well educated, and I've never heard this word before. So, it's not very  common. If you really want to use it, it might be good to explain it the first time you use it.

Yes, your alternative sentences seem more natural.