This is one of the best known walks in Australia, and certainly the one with the most infrastructure. The track has many kilometres of boardwalk and duckboards and there are huts, composting toilets and timber camping platforms at every recommended camping area. Not surprising really - up to 60 people per day leave Cradle Mountain (limited by a booking system) - and without the track hardening and huts the environmental impact would be severe.
I did the walk in peak season and was lucky enough to have no rain for the entire journey. In practice, the number of people on the track was in no way oppressive, and staying in the huts with about ten other people was actually pretty enjoyable. I'd rate it as a pretty easy walk, at least in summer and with good weather. The distances are short and the navigation is easy. When I did it, there was a family consisting of three generations on the track. I'd guess the kids were about eight years old or so.
For more information and details on booking a place on the track, see the official Overland Track website