Distance to the board?

One of my boys got into darts during the grand slam, since the worlds have been on the telly he's been bang into it, I bought him a board of his own and placed the trip 20 at eye height which is roughly where the regulation board reaches to on myself, how far back should he be standing? He's 130cm. He got his first 180 today, swiftly followed by a 140, he's 7 and already better than me.