What suit size would you suggest for a clothing size 10 slim?

My daughter is 9.5 she weights 65lbs about 4ft tall she wears size 8 pants She wears a size 26 in this suit. It fits nice. If I were you I would probably get a size bigger.

Can this be worn by women of all ages,ranging from the youth,to the elderly?

As long as the size fits and for the purpose of how you want to wear it. It’s a VERY tight fit. I bought it for young girls for swim competition. They complain it digs into the shoulders too much during long practices.

Is this item for plus size girls? Hence the name “big girls”?

No, this is not cut specifically for plus size girls. The size 20 fits my child 48" tall, approximately 47lbs. Thanks

What size would you get for a 10-12 normal clothes?

It depends on your child's built. Mine is petite so she wears a youth 10, the youth 12 is too long yet, even though she wears 10-12 children's clothing.