The youth academy has proven to be very controversial since its introduction in July '11, with many users unhappy with it in its current format; in general the main complaints are the remarkably poor youths that you receive with no investment, or inconsistent results with maximum investment. I've generally been happy with the results I've had out of the YA, although I've taken a very varied approach across all my teams and feel there are a number of things to consider. Essentially, the league format, market forces and the current state of your team financially can all influence your decision to invest, but the effects are completely different between official leagues, private leagues and the Xpert Ladies.
Official leagues
In the official leagues, the main influences on your decision to invest will probably be your requirement for younger players and your cash flow. It would appear that a number of teams invest in the YA already in the Xpert leagues, based on the low cost of youths such as players that are a 17/4 or equivalent. If you don't have a lot of spare cash (such as if you have a relatively new team) then you would probably be better to invest a smaller amount in the YA, keeping some cash back for training and purchasing youths on the market. If you have plenty in the bank, are more interested in SQs or are just feeling lucky, then you may wish to invest more - special qualities are definitely at a premium and, because they are randomly generated on youths, you may end up receiving a player that is of much greater value than most others of the same age and skill. Bear in mind that you are at the mercy of probability though, so don't bank on the YA being immediately profitable - it may take money to make money.
The Xpert Ladies
The Xpert Ladies offers one of the most unusual experiences of any league in the game, and this is all down to its unique transfer market. Since the Ladies has its own market, the supply and demand for players (and their subsequent value) fluctuate wildly throughout the season. During the closed season and the first couple of weeks, prices (particularly for young players) are incredibly high due to limited supply and large demand. However, as the season progresses, demand for young players drops very quickly (due to many managers being aware of the downsides of cashing out on transfer), so prices drop to record lows.
Investing a significant amount in the YA can prove beneficial here, but only if you're going to sell youths during the closed season when prices are high. If you receive youths later in the season, you may find that you'll have to give them games to boost their DV, or that potentially lucrative 16/4 may not sell and you'll find yourself with a much less valuable 17/4 on your hands by the time you can find a buyer. If you're investing heavily, it's definitely worth leaving a gap in your team for a potential youth and having a backup plan to train / play one heavily if you receive one at an inopportune moment. The unpredictability of the youth system does mean that you wouldn't normally want to rely on talents alone to build a team, but the Ladies is the one league where it's most likely to reap rewards.
Private leagues
The format of the league is one of the most significant things to consider when working out whether to invest or not in the youth academy in private leagues, although I'm not sure it's by design. Because teams can't receive youth players in the final 18 days of the season, leagues with short seasons will spend a larger proportion of time unable to bring in youths and thus lose some value on investment. The irony is that leagues with short seasons will be the ones that actual develop these players and bring them to maturity - in longer leagues it's definitely more productive in the short term to bring in established players to improve a squad and sell youths on.
In essence, you'll probably want to invest a larger amount in longer private leagues and a smaller amount (if any at all) in the shorter ones.
Other things to bear in mind
There are a couple of other points I'd like to mention here to try and cover everything. The first is that, while investing in the YA increases the probability of getting good youths (and, in the long run, you will receive better youths), you may still receive substandard ones. If you decide to invest in the YA, you will have to be prepared for the investment to not pay off straight away. Persist and you should see results over the longer term.
I've also mentioned in previous posts about the mechanism by which players can gain or lose skill when they transfer between clubs. If you're looking to sell a youth, then if they don't have any game time and you're a significant way through the season, then the player will lose some amount of skill. If you're close to the transfer deadline and it's a fairly long season (such as the 14 week official leagues), then that can equate to 0.3 or 0.4 of a skill bar. It's something that buyers can look out for in the official leagues, since it's possible to look at the player and team, but is not possible in private leagues.
Finally, your academy has to build up stars until it reaches your level of investment. It might not be the most economical way to take a 0-econ academy and invest 300k straight away, as it will be several weeks until your academy reaches 5 stars. I tend to build up the investment as the number of stars increase (i.e. invest 50k to 2 stars, then 100k to 3 and so on). It will take a little longer to reach the level you need but could save you some cash. Likewise, there may be the odd week where you can reduce investment slightly and save a little econ - you can maintain a 5-star academy by investing 300k for two weeks, then 250k every third week.
Finally, here are the levels of investment, with some details of how long it takes to get from one level to another based on my own investment data:
0 econ = ½ star
50 000 econ = 2 stars (3 weeks from 0)
100 000 econ = 3 stars (5 weeks from 0)
150 000 econ = 3½ stars
200 000 econ = 4 stars (7 weeks from 0)
250 000 econ = 4½ stars
300 000 econ = 5 stars (10 weeks from 0)
100k
2 star >> 3 star in 3 weeks
200k
2 star >> 3 star in 2 weeks
2 star >> 3.5 star in 3 weeks
2 star >> 4 star in 5 weeks
300k
2 star >> 3 star in 2 weeks
2 star >> 3.5 star in 3 weeks
2 star >> 4 star in 4 weeks
2 star >> 4.5 star in 5 weeks
