When conditions are right for them toadstools may appear in any lawn whether long established or recently laid. Generally they are harmless but they should not be eaten. Remove by picking them off (wear gloves), however if they are too numerous they can be mown off with a box on the lawn mower in order to collect them.
Following a good mowing regime should help reduce the frequency of appearance of toadstools. Also a sharp frost tends to reduce a toadstool population.