To be honest I never knew when to bring young young players into first squad and to be serious I never did that I jsut bought players and I want to ask when is appropriate time to bring young players into first squad and how?
When you think they are ready.
i usually rarely give players under18 the chance to be in first team. apart from certain 1-2 u18 in 1-2 league cup matches. but i usually take up a few from reserves as back-up and see which ones improve the more. i also make sure that they get the training they need. the tutoring from a first team player.
loaning out is the first thing i do, the first 1-2 seasons they will be loaned out so they get good experience.
but you have to be careful and loan them to a team they will be playing a lot.
you can play 1-2 friendlies as well in start of season and have some of your best youngsters with the squad and put them on mixed with first team and see which look to be doing well enough to be able to stay in the squad and improve as back up or if they need to be loaned out or not good enough.
but in the end u need to do well in ur coaching style with training and such things to make sure u get them 100% as well.
at times u might succeed to bring up 1 youngsters and fail with another.
The correct time to give your young players a chance is between July and August! For a manager July is a month of getting your team fitted and full of trials! The best is to give opportunities to 1-2 talented players, don't fill your First squad with yougsters! It's not a risk to put them in the first eleven during friendlies even for 90 minutes.. Usually most of the youngers are totally invisible, but there are some exceptions! That's how i work, and i tend to bring out at least 1 player per year at the senior squad!

Hope i helped!
besides red said, if you manage a team that plays in european competitions you need at least 4 players trained at the club so if you buy a 18yo(or above) player don't loan him more than one season otherwise you will lose the 3year training at you're club(in case he is an obvious future player for you're first squad). i usually buy 4-5 u21 players every season and one of them will eventually play for the first squad.
if you train them just right, sometimes send them on loan to a team and always put a senior player to touter them, you will get a great player.
So in my CS Louhans Cusineaux save in FM2010 and Im currently in season 15/16 and I have got player Christian Manco(18) which is regen and plays as CB and currently he has CA 79 and PA 118 and didnt play for any games but I have loan him to Vesoul HS which is semi-professional team to play for them.
that's good. even if he plays at semi-pro level, at least he plays.
Yea he plays and I hope in January transfer window to loan more young players.
Make sure the club you loan him out to has a few good coaches. I had a regen called Pantelis Papadopoulos, he was a striker, a wonderkid, and labelled the next Romario. I loaned him out to Stoke where he played most games that season.
Stoke don't have the best coaches and when I got him back he was no longer a wonderkid and he ended up becoming an average Premier League Player. He gets about 10 goals a season at mid-table clubs now... <sadface>
Too bad because I loaned him to a team which dont have any coaches. He is labelled (to a team-mate) as similar type of a player to Anderson
wiki
Here is picture with his attributes:
Personally I like to have a squad with a very young average age... at least half of the first team under 23. Like redarmy I don't often introduce players under 18 unless they already possess considerable current abilty. I'll generally introduce a young player with good potential with about 5-20 appearances a season and good training is important.
I don't really like loaning my young players out for experience, in previous versions I've found that despite them getting regular football on loan at another club, they rarely seem to make any improvement (dunno if this rings true for other FM players). I often find that young players improve more in my reserves on good training schedules than they do on loan.
I've noticed that determination, ambition and professionalism are 3 key stats for a young player to reach their potential, I've seen many young players with good numbers in these areas (all 10+) improve considerably over years without any first team football at all. I've also noticed that players with low ambition (5 or lower) will only improve with regular first team football.
Theres some of my thoughts on youth development... hope it helps