My 4 yr old got this with his Christmas money because he loved the Build & Play helicopter he had for Christmas. This kit is great value at £17.99. There are 6 things to build and it comes with clear, visual instructions. Build and Play is a lovely intro to Meccano - sturdy, flexible and easier for younger children to use, though it does take a lot of patience and will certainly need some adult help the first few times it's played with, and then probably occasional help with tightening screws, etc.
Regarding the age suitability, I think my son (who is nearly 5) would have found this one difficult if he hadn't already had the helicopter set. The larger sets not only have more things to build, but also more pieces to each vehicle. This set has 205 pieces and the digger uses most of them. So it might be easier for a younger child to start with one of the smaller sets to get a feel for how Meccano fits together and how to understand the diagrams of what to do, then move on to something like this. I'd say it's most suitable for 5-6 yr olds.