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Science doesn't fare as well for pessimists. They not only have lower levels of happiness compared to optimists, but research shows that people with negative thoughts are 3 times as likely to develop health problems as they age.
I am an ESL student. I can't get the meaning of 'as' here (in bold), and what part of speech it is. And also want to know if the sentence doesn't make sense without 'as', and if so, why. Please advise.
I'd say it was an adverb of degree modifying "likely". This is a comparative construction, in which the comparative clause has been omitted for some reason, although it's understood as something like as those who don't, where "as" is a preposition.
The adverb "as" is not omissible because it is in construction with the preposition "as". In full it would be something like:
... people with negative thoughts are three times as likely to develop health problems as they age as those who don't.
In your example, as is being used as an adverb used to emphasize an amount of something. In this case, it is referring to an amount of likelihood.
I would say it is an adverb of comparison as in "as big/tall as a house". In your sentence the second part "as those who don't have negative thoughts" is lacking because self-evident as BillJ already said.
You might even say "as" is a particle or function word. Much simpler as you don't to have to specify which word class the word "as" exactly is.
