I know this has been asked and reasked....in a recent trade attempt there was some discussion about a certain player 'already done progressing' at the age of 23.  Is there some formula guys use to determine this?  Or is it some kind of learned witchcraft kind of thing?

Do players continue to hold the ability to progress to age 27 as the rules state, with each individual doing it either faster or slower based on all the mitigating factors such as projected ceiling, adv scouting, coaches and playing time, etc?

Or is there really a foolproof way to determine if a young player is truly done progressing?  Is it a guestimate based on past leaps in progression (or lack thereof)?

This just seems to be a very nebulous topic...everyone has their own opinion...is there hard evidence pointing to either way?  I can put a link to the player in question if that is necessary but I am also interested in a general answer if there is one to development patterns.

It's rather simple.

You can see the players ratings from previous seasons by looking at the "Spring Training" lines of his profile.  Look from one season to the next, and the next, and the next, etc.  Then you also have the ratings from the current season, starting with "Opeing Day" and all the "Regular Season" updates.  You'll notice a pattern.  After a while they generally slow down and eventually come to a halt (or near halt). 

Looking at all these in total, it's fairly easy to see when a player has stopped progressing, or that he's just about there.

As to your other questions: ratings can be grouped into two categories: physical ratings, and skill ratings.  Physical ratings are a factor of age, training budget, and player makeup.  Skill ratings are a factor of coaching and playing time.

In general: physical ratings will tend to peak and then start to decline at a different time than the skill ratings.  So you potentially see some players who start to decline in some areas (power, speed, range, etc), while staying steady or even increasing in other ratings (contact, splits, etc.).

I would link the player in question in this particular instance but I dont appear to have the young kid computer savvy.

Every owner develops players a little bit differently.   There is some guesswork and some learned witchcraft.   I don't use a formula.

In short, I assume every player can develop into his 5th season.   Say he develops 8 points in his first full season.  I'd guess 4-5 in his second and 4-6 more before he's done.  However, I can look at the owners other players and say "Hmmm, he's doing a crappy job.   That 8 should be 10" and know, if I get him, I can develop him a little better than the 8-10 over the rest of his career.    

On his player card, you'll see a WifS logo.  Click it.   You'll get a "This player has been linked blah, blah, blah...."   Click the same logo on the header of the submit post column.    That's learned witchcraft.

Hardball Dynasty – Fantasy Baseball Sim Games - Player Profile: Ryan Page

Aha, witchcraft learned.

said another way in a very generic way (using OVR for simplicity)

If a guy started at 48, and has a projected of 75, and you see the following progression pattern by season

48
58
63
65
66

he will not magically have a season where he jumps from 66 to 72 ... he may get to 68 or 69 over the next few seasons, or he may be done now at 66, but from that development line, he is essentially "done" now, and thos projections are never gonna materialize

No, Ryan Page is not done developing.    Using VR, 6, 7, 5 and 1 so far this season.   This is also a good example of "different owners, different development".

Not sure if he had an ownership change but he did switch from NY to LA.  And he improved more(and pitched more) in his 2nd full season. 

I'd estimate him at 69-70 VR when he's done.   Sadly, he won't have the pitches to be a good short reliever.   He's likely to have 3 around 70.

My projected's show Cont95 vL95 vR64

I know some of that is a reflection on my adv scouting.  I just don't know how realistic it is for him to reach them.  So it is possible for him to overreach what I show for vR by the time he is done?

I show projected pitches of 78, 75 and 73

His VR is already 64.   69-70 is what I think.
Because of his "odd" development, I think his Control and VL might top out at 89-91.  
His pitches are going to be low 70s/upper 60s. 

Good information.  That said, if I acquired him could I give him the best chance to develop those pitches fully by making sure to have the best PC I can get for him? I.E., the environment he is playing under (coaches, training budget) can boost him or doom him further?  Or is his progression ticking along and nothing can influence it much?

Coaches, playing time and training affect progression.   Since he's in his 4th season, he's not going to explode but you can put him in the best situation possible to maximize what development he has left.     Other than VR, he's not going to reach the numbers you see.