This link
https://www.premierleague.com/news/58895
explains the Professional Development League quite well. Although it's actually got the regional split wrong. Once Brentford binned their Academy, Coventry went South and it's now 10 in each. It says three outfield players over the age of 23, plus the goalkeeper. The age group is defined as born after 1 January 1993. As far as I'm aware, as the Professional Development League & Premier League Cup are governed by the Premier League, it's a simple 3 subs allowed on system.
The Central League Cup is under the auspices of the EFL. I know they allow 5 subs in that cup. I did not know that beforehand. I only knew that when Rotherham brought their 4th & 5th subs on against us.
http://ball-sup.blogspot.co.uk/2017/01/full-blown-right-hander.html
I also know (because they told me) that Binnion & Dungworth didn't realise they could make 5 subs until they saw The Millers do it. The lack of attention to detail that our great club is renowned the world over for. Because that Bradford game you mentioned was a Dead Rubber, they decided to use all 5 subs in that one. (Now that they knew you could ffs). And got arses burned a little bit when the keeper Eastwood went off after all 5 subs had been used.
http://ball-sup.blogspot.co.uk/2017/02/wilder-priority.html
I'll use your question to plug...... Every week, the Premier League publish two PDF Bulletins. One covering the U18 level, one the U23. Results, league tables, fixture changes & upcoming fixtures - including times & venues. It's the Carrier Bag Firm Bible.
https://www.premierleague.com/publications
scroll down until you reach the section headed Youth.
The EFL do one for the Central League as well.
http://www.efl.com/global/reservesnews.aspx
And there is an excellent website dedicated to Youth Football, with up to date results & tables etc.
http://youthhawk.co.uk/wiki/Main_Page
While all of those links are worth following, they all seem to have an appalling lack of information about microbreweries, beer ticking, craft beer bars & Hop varieties. So, I hope you'll remember our sponsors at...
www.ball-sup.blogspot.co.uk
and
twitter.com/ball_sup