Finding files with extensions using find command and files which are 90 days old

find /path/to/search -mtime +90 -type f \( -name "*.txt" -o -name "*.pdf" -o -name "*.doc" \)

This will find all the files in directory /path/to/search which are 90 days old and files having extention as txt, pdf, doc. 

Like this you can add many extention and specify time accordingly to find specified days old files.