Would my 13 year old son weighing 60kg be able to handle a dart 16 ?

I have never seen a Dart but I looked up the specifications and it looks ideal as a youth boat, no boom to conk heads, kick up rudders so no panic, no centreboards and tough poly construction. When I ran a junior program I felt 14ft cats were plenty fast enough but they had centreboards and booms. If your son doesn't go out in heavy wind he should find it a lot of fun

I see from your query on the Aus forum you are considering the smaller Nacra's heres a link to a site in the UK that gives a contact phone number
http://www.nacra.co.uk/Nacra_UK.html
This is a good video showing some of the 3.5's and 4.3's mixing with the big boats at the last Nationals
https://youtu.be/Ujo0-KzMYk0
good luck with your search I sailed on a Nacra 4.3 last year and was great fun, at the nationals in this video there were 15 smaller Nacras racing on the same course with us 40 bigger boats, these boats had all combos, some just a boy, some just a girl, some mother daughter combos and some father son, one 3.5 had an adult male on it who normally sails on an F18 Infusion to a high standard. They are incredibly buoyant boats with the same mast section and beams as a Nacra 5.8 so really robust construction