No children?

Are you sure it's not a 55+ community? Typically the by-laws can't ban anyone, it would need to be a covenant.

Typically the restriction on under 18 year olds living there is a result of the agreement the developer made with the local government. The reason being is that what they were saying is there will be no school age children here thus not a burden on your schools/taxes so please approve what we want to do. Many towns would jump on this proposal as they get the taxes and do not have to provide schools.

I believe The Master Deed is the same as Deed Restrictions or Covenants. When one buys, they agree to abide by such. Sometimes they can be changed by the majority of members/owners agreeing to change such. Sometimes it takes all (100%) of owners agreeing. I also believe there could be some Deed Restrictions that cannot be changed.

I can pretty well assure you that many bought there knowing the under 18 restriction existed and they will not want to change it. Many wanted there to be no rug rats living there. Now a few young 18 year old, good looking girls are another story. No school busses for them but some of the old guys in the neighborhood will gladly take them to school....especially when the weather is hot....LOL

Bylaws are more exacting descriptions of how the association will be operated like how one gets elected to or kicked off the BOD. Deed Restrictions override Bylaws.

It's illegal, a violation of federal law, The Fair Housing Act. Just Google HOA discrimination against children.

Okay, according to the Fair Housing Act:

Housing Opportunities for Families

Unless a building or community qualifies as housing for older persons, it may not discriminate based on familial status. That is, it may not discriminate against families in which one or more children under 18 live with:

A parent
A person who has legal custody of the child or children or
The designee of the parent or legal custodian, with the parent or custodian's written permission.
Familial status protection also applies to pregnant women and anyone securing legal custody of a child under 18.

Exemption: Housing for older persons is exempt from the prohibition against familial status discrimination if:

The HUD Secretary has determined that it is specifically designed for and occupied by elderly persons under a Federal, State or local government program or
It is occupied solely by persons who are 62 or older or
It houses at least one person who is 55 or older in at least 80 percent of the occupied units, and adheres to a policy that demonstrates an intent to house persons who are 55 or older.

According to HUD's website, you can file a complaint and the federal government will fight it for you. It appears that they take it seriously as discrimination and an important civil rights issue. There is a complaint form at https://portal.hud.gov/FHEO903/Form903/Form903Start.action , you can talk to a specialist at 800-440-8091, and they even have an app for a making the complaint.

This case seems pretty easy for HUD, because the discrimination is in writing in the bylaws. You don't have to wait until after you buy, since the law prohibits discrimination when trying to buy.

However, before making a complaint, I would contact the HOA and give them a chance to do an internet search for themselves. I think reasonable people would agree the change an illegal rule when faced with government opposition.