I can answer this one from direct experience.
My aunt used to give me M&S vouchers for birthdays and for Christmas. I recently found a total of £35 worth of them in a drawer. Knowing that an elderly friend of mine loves M&S shopping, but preferring cash myself, offered the vouchers to her for £20. I told her that all of the vouchers were at least 10 years old, but some were probably closer to 20 years old, so I gave her the vouchers on the understanding that she'd only have to pay me if she was able to use them.
My friend phoned the Ipswich branch of M&S and was told that they couldn't see any reason why the vouchers couldn't be used. When she visited the store she found that the more recent vouchers could be recognised by the electronic tills but a manager had to be called to allow the older vouchers to be processed. However M&S did honour the full value of all of the vouchers.
If your vouchers are only 6 to 7 years old, they're almost certainly of the newer type (which can be scanned at the tills) but, even if they're not, they're still valid.
Chris