Hi Liz,

First of all, see this page:
http://www.unicef.org/lac/flash/DW/dia_de_la_juventud.html

For some reason, I can't copy it directly, but in the 5th para it quotes the U.N. as "generally considering "joven" as an individual between 15 and 24.

Considering that your organisation focuses on kids up to the age of 18, I would say that while your text DOES MEAN to imply "older children" when using the term "jóvenes", it isn't really necessary and isn't precise either.

If you use the term "adolescent" you are automatically including any person up to 19 years old and, if you so wish, can always use older adolescent. In the cases where all age groups are mentioned I certainly don't feel that you need to underline "older adolescent". Consider the following:

The official UN working definition of "adolescent," as set forth jointly by UNFPA, UNICEF and the World Health Organization, is the age group 10-19
http://www.lifesite.net/ldn/1999/feb/990212b.html

The United Nations General Assembly defined ‘youth’, as those persons falling between the ages of 15 and 24 years inclusive. All United Nations statistics on youth are based on this definition
By that definition, therefore, children are those persons under the age of 14. It is, however, worth noting that Article 1 of the United Nations Convention on the Rights of the Child defines ‘children’ as persons up to the age of 18
http://dancolombia.blogspot.com/

I haven't found the U.N. to be quoted for a definition of "young adult", but I would usually understand it as a synonym of youth, wherefore you'd be targeting an age group that is different to the group your organisation works with.

Therefore I'd go for either of the above and, where you feel it is absolutely necessary, you can specify "older adolescents/teenagers" considering that "teenager" automatically implies 13-19.

Finally, if you still feel uneasy about the terms, I wouldn't hesitate to ask the organisation. Although they might focus on under 18's, this might not completely exclude young adults or they might have their own particular reasons for using the term and thus want it to be conveyed in the translation.
This is something that you simply cannot know - after all, you're a translator, not a mind reader!

Good luck!

Álvaro :O) :O)

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Note added at 1 hr (2007-10-30 12:11:05 GMT)
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Hi Liz. Sorry, I'd spent some time writing that answer and hadn't seen your comment. Personally, I couldn't agree more - this does seem to be an extra effort to appear inclusive. However, as I said earlier, if you feel uneasy about it you can always ask. :O)