This of course is dependent on the individual developer and what goals they want to accomplish. But I think the only real measure is the development of games! But then what are the milestones for a game? There are simple enough games that they could be created in a single day and there are those that take years (into mine over a year already!). Clearly the span of creation is a large factor for milestones. If you're not going much further than a mile, you don't pass that many stones :). That being said, there are probably some generic milestones for the medium sized game, which would certainly put someone on track to rock-star game developer status.
(damn this auto numbering! I want to start at 0 like any good programmer would!)
Get an idea for a game. Easy, who doesn't have a list of those somewhere?
Choose your platform and language. You'll always see this question on this site. It's a big one for new developers. (But don't ask here because it's off topic! This is one challenge you need to overcome on your own)
Write the first line of code for your game. As simple as it sounds, this is a fairly big one. I bet there's 100 people who have an idea for a game they want to make but never start for each 1 person who completes this first milestone. They get stuck during a tutorial or they get so caught up in thinking about how to do it, they never do it.
Get something basic going. This could be as simple as getting a triangle to render on screen or using the keyboard to move a little sprite around. At this point you have a bare bones game. It starts, displays some graphics, responds to the player and then ends.
A million smaller milestones These range from getting your art assets loading correctly, to fixing that damn bug, to reading and writing to disk, to that premature optimization that was bugging you just too much to leave it. Getting past this collection of milestones is a pretty big milestone. Plenty of people will give up at this point or start working on something else and never come back. The majority of the milestones in this milestone involve code change->play->repeat. It's important to play test often!
A playable prototype. This should represent your finished product fairly well. Now things are starting to come together. You're 90% there right?
A million million smaller milestones. Oh the grind. You're deep in your second 90% with lots of little changes, fixes, re-optimizing your premature optimizations, brain meltdowns and code re-factoring. This is a major stopping point as well. This part is deceptively difficult. Broad strokes are easy in comparison to the fine art of polish.
A complete game. Now you have 3 grey hairs and a finished product to show for it. Congratulations! Now you just have to publish, promote and maintain.
But really the milestones are a lot like this list. Made up on the spot and totally up to you.