Short answer. Yes. At 18 you reach the age of majority and are emancipated, which by law makes you qualified and responsible for all acts of civil life. You may enter and sign contracts on your own, without a parent co-signing, subject only to a few special exceptions.
What the law says
qualified and responsible for all acts of civil life, save the exceptions established by existing laws in special cases
RA 6809, Section 3 — Effect Of Emancipation. Read the full provision →
What turning 18 actually changes
Reaching eighteen does more than mark a birthday. The law provides that emancipation shall terminate parental authority over the person and property of the child, who is then treated as an adult in dealings with the outside world. From that point you no longer need a parent or guardian to act for you or beside you. You can open the transaction, negotiate its terms and put your own signature to it, and that signature binds you the same way an older adult's would. The days when a contract you signed could be undone simply because you were a minor are over — your youth is no longer, by itself, a reason a deal can be set aside.
The one big exception: marriage
Full capacity to contract does not mean full freedom in every decision. The same provision keeps one important brake in place: contracting marriage shall require parental consent until the age of twenty-one. So an eighteen-year-old can sign a lease, take a job, or buy on installment alone, yet still cannot marry without a parent's consent for three more years. Marrying between eighteen and twenty-one without that consent does not make the union void, but it carries consequences the law attaches to the missing consent. This is the clearest sign that emancipation gives you civil capacity, not a blank cheque over every act with legal weight.
What still limits a young adult
Your new capacity is granted save the exceptions established by existing laws in special cases. Certain transactions carry their own age, licensing or formality rules that an ordinary contract does not, and those special rules still apply to you. The law also does not fully cut the cord for responsibility: parents and guardians can still answer for certain wrongful acts of those below twenty-one who remain in their care. So being contractually capable at eighteen is not the same as the law treating you, in every respect, exactly as it treats a person of thirty.
Signing wisely, not just legally
Because your signature now truly binds you, the practical caution matters more, not less. Read the whole document before you sign, keep your own copy, and make sure any promise made to you in conversation is written into the contract itself. If you cannot pay, or the other side does not deliver, you carry those consequences as an adult. A contract that looks one-sided, hides fees, or pressures you to sign on the spot is worth pausing over. If the stakes are large — property, a loan, a long commitment — have a lawyer look at the terms before you commit rather than after a dispute has already started.