If you were born in 1955, you are a baby boomer. If you were born in 1985, you are a millennial. 1985-1955=30. 30 isn't the normal age for a grandparent.
Latest possible boomer is early 60's. I took pretty much smack dab in the middle of the boomers. Millennial starts in what, '81-82? so about 16-20 years between, which is generation X. The earliest possible boomer is early 50's when the latest millennial is born. The latest possible boomer is 19 when the earliest millennial is born.
If anything, millennials would be their grandchildren.