I'm done with high school, working full time and want to move into a house with 4 girls. I turn 18 in August but does that mean the contract is voidable or does it just become valid when I turn 18?
The contract is voidable - but someone has to raise the issue. The best option may be to have someone older co-sign with you. The landlord is looking for responsibility for the rent payments. If no one raises the issue, it probably will not matter so long as you keep paying your portion of the rent.
It means it is voidable because you are a minor.....but landlord can take the risk. More likely that the landlord makes your Mom/Dad sign for you.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.