the game itself doesn't have any age requirements, but think of the following factors and how they could influence the young mind before making your decision;
a) the influence of others in the game - most are teens, who'll swear, say inappropriate things, and occasionally hit on this person
b) the influence of the game itself - its a 'neverending' game, can this person deal with proper time-management? Is his/her studies going to go in the dump for later years as he picks up the habit of such addictive video games now?
c) game mechanics themselves - its rather time consuming to really get anywhere in RO - I would think that anyone less than 10 years old would quickly become frustrated with that, as they probably dont have the patience to lvl up and 'grind' or farm for money.
in the end, you'll have to use your own judgment to see if its appropriate or not for this person to play :/ I know I'm not letting my 11 year old sister play till she's 15+, because of a and I don't believe she can handle b.
RO is T in the ERSB for fantasy violence and language, so age's 13 and up ( if anyone cares >.>)
Also ERSB doesn't rate online interaction
anyone can play
lol.... 
@lil_dragon_blue : Thanks , that was great, gave me a bunch `o ideas

Time management is another thing, never thought about that

, probably the hardest to comply with along with youth and young ones

.
@Asguard : Yeah alright, in the first place I thought it was age 13+, that was helpful, really re-assuring lol

@portjack : " Anyone can play " even a baby?!, Can the baby even climb up the chair if its a desktop computer? Can the baby level up?!
What a disaster!!, I ish not letting my baby play ( If I had one o.o)
Portjack,

anyone can play XD