After leveling your Juvenile mount to level 5, your mount name should change to Adult, instead of Juvenile.
Your mount should look something like this when it becomes an Adult mount.

When it becomes an adult mount, you don't feed it
magic powders, you feed it Forages. You can buy forages from
mounts keeper, for 2.5k, 5k, and 12k each. Or you could buy Condenced forages from
MWO Mall.

Adult mounts have a total of 10 levels.
Every 5 Levels, your mount will have a 50% chance of getting a socket to put amulets in.
And every 1 Level, your mount will have a 10% chance of getting a Hidden skill, which can be casted by mount with a 10% chance.
I used an amulet with the socket already...
it's the Singular Resistance.

My precious mount "Kodophant" is right now level 9, +99% EXP with 1 Hidden Skill, and 1 Socket. Next level i have 1/2 chance of getting a socket, and 1/10 chance of getting a skill 

Let us pray for it to get BOTH! Socket and Skill!
LOLLOLOL it turned out as this..
I thought i wasn't going to get anything, no socket, no skill. But yea, atleast i got another socket now

Well... not really a guide... only half of it is 
Tag: Mount Adult Guide