It is quite true that the tendency in the UK (and in the US, for all I know) is for messages on this subject at points of sale to emphasise the prohibition on sale to minors rather than on consumption by minors. Both, by the way, have been illegal in Spain since December 2010, since the "Ley 11/2010, de 17 de diciembre, de prevención del consumo de bebidas alcohólicas en menores de edad" was passed the PSOE:

" Artículo 12 Limitaciones al consumo, venta y suministro
1. Queda expresamente prohibido el consumo de bebidas alcohólicas por menores de edad.
2. Queda expresamente prohibida la venta y suministro de bebidas alcohólicas a menores de edad."
http://noticias.juridicas.com/base_datos/CCAA/ga-l11-2010.t2...

But this message refers to consumption, not sale, and I don't think it's legitimate to change the actual content in translating it. I agree with Lisa on that. However, I do think it's appropriate to adapt the style to what would probably be said in such a notice in an English-speaking country, and I would suggest this.

"It is illegal to consume alcohol if you are under 18 years of age"
http://www.musclebeer.co.uk/products.html

You could include "years of age" but it's not really necessary. I think it has more impact without.