because the word is 'much'. I mean, you can use 'much' in stead of ' very'.
MUCH makes the meaning stronger. 'I am much older...' is better.
VERY has different usages.
AS...as is a pattern which is used when two persons or things are equal in amount or degree.
Mary is hardworking. John is hardworking, too.
JOHN IS AS HARDWORKİNG AS MARY.
Here, we use the adjective in the middle of two 'as' ses.
When you want to compara two people or two things, you can use Comparative.
If the adjective is one syllable, add -er + than
Ali is quiter than his sister.
I am older than my brother.
She is younger than her sister.
The Antarctic is (much) colder that the Equador.
REMEMBER: We are better at English when (if) we can produce sentences like the English do. and English say it that way.