Level 20 character, actually:
Ranger 2/Paladin 3/Beastmaster 2/Halfling Outrider 10/Wild Plains Outrider 3
feats: Devoted Tracker (Complete Adventurer), Dragon Steed (Dragonomicon), Mounted Combat (PHB), Mounted Archery (PHB)
You have:
Effective Ranger level 12
Effective Druid level 15 (Beastmaster)
Effective Paladin level 16
Total is Druid level 21 (Ranger is 6 equivalent plus BM "Druid" levels), Paladin Level 16
You get a Very Young Gold at Paladin level 16 with the usualy 5th level Paladin goodies (+2HD, +4 NAC, +1 Str)
Druid animal companions get another bump at level 21 (every 3 levels after 18 as per Epic book) (so gets +14HD, +7 Str/Dex, +14 NAC, etc.)
A Very Young Gold starts at 11HD. Add the above, and you have a 27HD Gold Dragon plus a bunch of other things. It is 100% legal (and not that powerful, in any case, as your PC will end up being a Fighter with no feats, less average HP, etc.).