Añitos
 

Soy un estudiante, 18 añitos...

I know añitos is a diminutive of años--what is the shade of meaning?

That "un" makes it seem to be sort of a part of a personal profile. "Añitos" can imply many things in that context and give dozens of nuances in different contexts. For instance, a party girl can say "18 añitos" to say "age, but Lolita like"; other person can say "18 añitos" because his or her birthday was last week -they are "new 18"-. The range can be pretty wide and wild.

I agree with Alec, although around here, "añitos" usually means that someone is rather young, as if their lived years were shorter (smaller) than adult's ones; the idea is just that they have lived less years than older people. :)

What if they said 60 añitos. Does that mean like "I'm 60 years young"?

It would be an ironic way of saying "old", but usually by saying "tengo unos cuantos añitos" or "unos cuantos añitos más que tú" as a way to say "tener colmillo" in Mexican style; or it can be a statement of still having a lot of rails to road on. These "añitos" have always to be in context.

Probably... It might mean "I'm barely 60 years old", and they would obviously be joking.
However, a 60 year old person here would be more likely to say "añotes". :D
The idea of the diminutive/augmentative is to underline less experiences or more experiences in life, according to the one who talks about it. :)

Mi hijo ya va a la primaria. Tiene seis añotes. -> My son is already going to primary school. He's already 6 years old.
Mi hijo acaba de aprender a leer. Apenas tiene seis añitos. -> My son has just started to read. He's just 6 years old.

Ya tienes treinta añotes. No deberías vivir con tus padres. -> You're already 30 years old. You shouldn't live with your parents.
Mi abuelo me dijo: "Apenas tienes treinta añitos. Te quedan muchos años más por vivir." -> My grandfather told me: "You're barely 30 years old. You still have many more years to live."


I also agree with Alec's ironic use of "añitos". :)

The girl is telling you that she is not too experienced. :)

And like you have been told about the 60 years old, is trying to pass as an unexperienced (jokingly, sarcastically etc.)

Well, in some countries they tend to use lots of diminutives in the daily living. It's just to sound informal.

Ej: rapidito, calladito, niñito, chiquitito, ratito, cortito, etc.

Especialmente en Chile, no? :)

@Hernán: ...y en México... a veces se vuelve desesperante oír cosas del tipo "m'hijito, dame la manita para que te ponga tu ropita..."