I'm posting this because I know you will appreciate it ...
My house is 10 years old. For the first 5 years it had tenants and we've owned it for the last 5 years. My floor grout is super sad, regardless of how much I'm mopping it. I'm currently doing the big Christmas clean and I've tackled the kitchen floor (with the rest to follow). The bestest floor grout cleaner ... toilet cleaner (with bleach in it). I used Harpic White and Shine with thick Bleach Gel. OMG!!!
Process - I just ran the spout along the grout lines. By the time I got to the end of the kitchen, I went back with my tile brush (which I bought from Woolies) and ran the brush over the grout a couple of times. Then when I got to the end, I went back to the beginning and went over it with a sponge, wiping the cleaner over the full tile. (This whole process took 15 mins). I then used a dry mop to mop up the cleaner, then washed the floor a couple of times to make sure there was no cleaner left on the tiles. The whole process took 1 hour - mopping took the longest.
I'm attaching some photos ... please don't be too harsh in your judgement of the "before" photo. Didn't take an entire floor shot before because I'd already started when I took the before photo. They look as good as the day they were laid (I would imagine).