Many mentions upthread of to-scale
orrereyeyeys scale models of the solar system. If the models of the inner planets are visible to the naked eye, the resulting system won't fit in the child's
bedroom house. See
http://thinkzone.wlonk.com/SS/SolarSystemModel.php with Mercury at 0.1mm diam, the radius of Pluto's orbit is 149.8m
Or make a model yourself
Scale Model of the Solar System, Scale = 1:14,000,000,000
planet scale size perihelion aphelion
The Sun 10cm diameter 40 watt bulb 0 0
Mercury 1/3mm paint drop 10 3/4 feet 16 1/4 feet
Venus 1mm paint drop 25 feet 25 1/2 feet
Earth 1mm paint drop 34 1/2 feet 35 2/3 feet
Mars 1/2mm paint drop 48 feet 59 feet
Jupiter 10mm white marble or bead 173 feet 191 feet
Saturn 8 1/2mm hatpin or pearl 316 feet 353 feet
Uranus 3 1/3mm map pin or pearl 642 feet 705 feet
Neptune 3 1/5mm map pin or pearl 1045 feet 1063 feet
Add in their moons using fuse wire and pins with further small blobs.
Alpha Centauri will be another light bulb, 1800 miles away which is somewhere between Istanbul and Baghdad.
(Copied from
http://www.essex1.com/people/speer/model.html )
More seriously, age recommendations are at best an indifferent guide. Best be guided by a combination of instinct, knowledge of the child, and - if not the parent yourself - liaison with the parents.