Hi there, Natan!
The common way is to say (if someone asks) I am xx years old.
(where xx is a number). If you are writing (I mean, academical writing) you should write it all, or put the number, as in: I am 99 years old
In informal ways, you can find it pretty much in any way, sort of. But in sites, which are a mix of informal vs formal settings, I would say a sound practice to refer to someone´s age would be in the "I am/she is/he is xx years old" form (when for the first time), then it could be stated just in numbers, as in:http://www.gq.com/story/how-old-is-brie-larson-tho
Larson was nineteen when The United States of Tara
Larson's character in Scott Pilgrim vs. The World
, was 22-year-old Scott Pilgrim's girlfriend from college who went on to become a rock star after their breakup.
Larson played the 18-year-old
love interest of a downward-spiraling 35-year-old
(Michael C. Hall). (notice, when the mentioning of age is in the same sentence the author might use the same form.)
Larson was 25 when The Gambler
...beleaguered parent of a five-year-old boy
In some chat-rooms (most of them) you can use, e.g. "I am a 20 y.o. Guy" (meaning I am a 20-year-old
And, as an aside note, as far as I know the grammar doesn´t change much when it comes to the two brands of English, specially when talking about age.
Granted. It would be different with telling the time!