I think that’s probably general “opinion” rather than “advice”. I doubt there’s much evidence – you tend to get more float with SPDs than you do with pinned flats anyway. You won’t see many under-12 road racers not clipped in, nor that many under-12 off road racers for that matter.
I’m not pushing one way or the other, but there are some good advantages to being clipped in. My eldest has been on SPDs since he was 7 – that was what he wanted, as a light rider on small wheels (trying to ride rough stuff fast) he was getting bounced off flats too easily. Now (10) he rides both flats and SPDs, but he wouldn’t ride flats on the road… the shame 😉