Do you native speaker think you need context to interpret 'a couple', 'a young couple', 'an elderly couple' as a married couple?
You mean for all those three?
Yes, for all three.
On their first date, the couple went to dinner and a show. (they are not married)
The young couple were overheard planning their elopement. (they are definitely not married because they are still in the planning stage)
The elderly couple were brother and sister. (self evident)
If you want to say that any of the couples are married, you need to provide sufficient context so that the reader knows that they are married.
Yes- age doesn't change anything and couples don't need to be married.
Good. Just as I thought. Thank you both!