OEM batteries are all pretty good quality, but to be fair a six year old battery is living on borrowed time, I would not complain about a battery being weak at that age, as batteries age they lose both the ability to store and release electricity. There is a reason that standard chemistry lead acid car batteries only get a 3 year warranty.
Get a new Yuasa battery, buy a 'silver' if you have the money (about £70), you will get a 5 year warranty, then you can have the radio or whatever on as long as you like - and it may well give good service for 6 or more years.
Having been involved in battery manufacture I never leave a battery on a car more than 5 years, there are so many things that can affect its life (high temperature, has it ever been allowed to go flat etc. etc) that its a lottery after that and batteries are not that expensive - it is called preventative maintenance.
Battery capacity diminishes so slowly over time that it goes unnoticed until one day it does not perform (normally in winter, but it is heat that kills them) and people blame bad quality, we are lucky in UK our batteries last a decent amount of time, the reason there are so many battery makers in USA is that in their hotter states battery life can be as short as 18 months.