"Life is too short."
Translation:La vida es muy corta.
Demasiado can be an adjective('too much' or 'too many') or an adverb('too'), it only agrees with a noun when it is an adjective as in 'demasiadas vacas' = 'too many cows'. When it is the adverb 'too', it is invariable so is always 'demasiado' even if the adjective/noun ('vida corta') is feminine
It doesn't actually matter what gender the word is. If you go back up to the first post on this page, you can see it explained well. Here's my version:
"Demasiado" can be either an adjective (modifying a noun or pronoun) or an adverb (modifying a verb, adjective, or another adverb). In this case, it is used as an adverb, modifying "corta," as ProfesoraWright so wisely pointed out. And, as ricrog (above--first comment on the page) explained, demasiado only changes to match gender and number when it is an adjective. The adverb is always, always, always "demasiado"--singular and masculine.
Kind of like in English, we wouldn't say "Toos much cows," because "too" (an adverb) is always singular.
Hope this helps a few people. ^_^
I agree with most of your explanation. But if I can clarify two points. Demasiado doesn't change as an adverb because adverbs have neither gender nor number. So, although demasiado is the same form as the masculine singular adjective, it is neither masculine nor singular. It is an adverb. Secondly it is misleading to make any comparisons between Spanish and English agreement of gender or number. In English neither adverbs nor adjectives agree in gender, as there is none, OR in number. So your toos many cows argument is specious.