Hmmmm ..... that is certainly a rule in the under 23 Premier League in this sense. There can be a maximum of 4 over age players on the pitch at any time plus the goalkeeper can be any age, which is probably used a lot. There are a lot of big clubs whose best 3 goalkeepers are over the age of 23. So a team could play 3 over age goalkeepers in 90 minutes, though a goalkeeping substitution actually counts as a substitution in the overall total.