To add some food for thought, Zecora explains that a Dragon is capable of growing at a fast rate based upon the size of his horde and if he allows greed to control his heart. While this seems to be a fast-track to raw power that lacks the years of mental development and skill gain (Spike went from small but intelligent, and capable of talking, to massive but somewhat slow, and incapable of basic speech), it does suggest that a dragon left all on their own from the moment they are hatched can occur, and that the greed instinct is a way for such lone ‘baby’ dragons to level the playing fields and survive.
 
Spike underwent this change, but the change was reversed the moment he remembered the importance of being generous, thus purging greed from his heart. By turning his back on the greedy instinct, his horde became irrelevent as he did not care about collecting and protecting it anymore, which returned him back his original size.
 
When he encountered the ‘teenaged’ dragons, he never underwent any physical changes even as he mimicked his peers. But he did temporarily earn their respect from them and become accepted in their group. It’s clear that the ‘teenaged’ dragons have better mental development and greater skills than Spike when he was huge. So while they are growing at a slower pace, they are retaining and probably increasing their overall intelligence and range of skills by watching out for one another.
 
It seems that mental and physical growth in Dragons needs to be balanced carefully in order for a Dragon to reach full maturity with the best physical and mental capacity they can possibly attain. If they grow too fast, their mental abilities become suppressed, and may or not return years down the line. Conversely, Spike is much more intelligent and mentally mature than the ‘teenaged’ Dragons (making him very ‘mature for his age’), but is far smaller in size, so it is possible that rapid growth of mental strengths can impede physical growth.

Thinking on it, it might be that the gain mental maturity primarily to keep from getting killed by older, larger dragons as they are on the whole a rather violent species. A further thought is that the mental abilities they gain fade as they grow in size as they are needed less, which is why Spike got dumber as he grew during SoME, only to return when the dragon reaches full maturity.

@Candrew  
yes dragons most likely don’t have a concept of family, their ‘mental maturity’ may not be all that relevant though dragons are pretty tough creatures and would probably be fine without the concept of maturity at least how I’m thinking of it; in fact the dragons that spike interacted with seemed very immature even though they were older  
They probably do normally gain independence early on though which is why they made fun of spike he seemed to still need someone to look after him so in dragon culture he was percieved as weak  
Spike being somewhat mature is probably just a result of being raised by Celestia

@Candrew  
I know they aren’t real, (derp) but seeing how they are reptiles, it’d make sense if they were abandoned just like that. Unless they were more akin to gators and crocs, then they’d have at least a few years of childhood for some kind of maternal and paternal (gators) protection.

My point was more that, while he is certainly a young dragon, the phrase baby dragon was one Celestia created to keep ponies from freaking out and is not actually something that is an actually classification of dragon development. Garble and the others mocked him for being small, but they never made fun of him for being a ‘baby’ dragon, just a shrimpy pony made one. I’ve always assumed that dragons matured mentally first, and physically second, and given the lack of involvement adult dragons seem to have with the young ones that is probably a defense mechanism to help the younger ones survive. I wonder honestly if dragons even have a real concept of family that extends beyond the first few years of life.

@candrew  
I figured he was a baby comparitively because dragons live thousands upon thousands of years and he’s like 10-15 and dragons ‘mature’ after they’re like 50-100 (of course this is headcanon seeing how little we know of Equestrian dragons)

The thing is, though Spike and Twilight are friends, she’s still technically his boss. I don’t disagree that their relationship is strained, but I think a lot of that is difficulty changing long held perspectives. I also get the impression that Celestia would rather Twilight see her as a friend first and a teacher second but changing long held relationships is hard.
 
On a side note, I always assumed, and that fact that no pony seems to know anything about dragons seems to confirm this, that Celestia made up the term ‘baby dragon’ to keep ponies from freaking out over an apex predator living among them by having them focus on the ‘baby’ part rather than ‘dragon’. Considering most fan’s ideas about his age I’d say it worked.

@Impious  
Owl’s Well that Ends Well doesn’t count? It’s sorta like that.

People say that a lot, but I never see any evidence of it. He’s an assistant, he assists. The includes cleaning and cooking. If he didn’t t least consider her a friend I don’t doubt that he could have left at any time.

Twilight barely acknowledges Spike, though. That really needs to be an episode with permanent consequences, with the lesson “never take those who care about you for granted.” Spike would leave for greener pastures, for good, and the episode would end on a somber note with Twilight deciding to visit her parents for once.
 
Seriously, he’s not much better than a slave, and despite having the mind of a young teen he is constantly called a baby. That kind of belittlement a positive relationship does not make.

Spike was more-or-less her slave though

Plus, she had Spike who was certainly friendish is nothing else.

Having her brother be her only friend is a strong indicator of her not being good at making friends.

Twilight’s only friend is a family member… Exactly like Celestia, and at the time she couldn’t even say that much. Maybe Celestia just didn’t want Twilight to end up like her. And, perhaps the friendship reports aren’t just meant for Twilight.