Short answer. Yes, in limited cases. Adoption is not only for infants. The law allows a person of legal age to be adopted where, before the adoption, the adopter already treated that person as their own child since minority. Note that adoption is now an administrative process under Republic Act No. 11642 (2022).
What the law says
A person of legal age if, prior to the adoption, said person has been consistently considered and treated by the adopter(s) as his/her own child since minority
RA 8552, Section 8 — Who May Be Adopted. Read the full provision →
Adoption is mostly, but not only, for minors
The general picture is that adoption is for children — a person below eighteen who has been declared available for adoption, or a spouse's own legitimate or illegitimate child. But the list of who may be adopted is wider than many people assume. It reaches an illegitimate child being raised to the status of legitimacy, a child whose earlier adoption was rescinded, and a child who has lost a biological or adoptive parent. Sitting inside that list is a genuine exception for grown-ups, which is why the answer to whether an adult can be adopted is not a flat no. It depends on the relationship that already existed between the two people involved.
The adult who was already raised as your own
The specific window for adopting an adult covers a person of legal age if, prior to the adoption, said person has been consistently considered and treated by the adopter(s) as his/her own child since minority. The key is history, not the adult's present age. If you took a child into your home, held them out as your son or daughter, and cared for them as your own from their younger years, the law lets you formalise what has long been real, even though the person is now an adult. What it does not do is let a stranger adopt any consenting adult on request — the lived parent-child bond stretching back to minority is what unlocks the door.
What has changed since this rule was written
Read this subject to a newer law. Republic Act No. 11642 (2022), the Domestic Administrative Adoption and Alternative Child Care Act, moved adoption out of the courts and into an administrative process handled by the National Authority for Child Care. That statute restated the qualifications, the required consents and the eligibility rules, so the procedure a family follows today is governed by RA 11642 rather than by a court petition. The lasting effects of an adoption survive the change: the adopted person is treated as a legitimate child, with the reciprocal succession rights that flow from that status. The path has changed; the destination has not.
What adopting an adult actually accomplishes
A completed adoption creates a full legal parent-child relationship where before there was only affection or custom. It gives the adopted person the standing of a legitimate child and the inheritance rights that come with it, and it binds the adopter to a parent's duties. It does not erase every prior tie or obligation, and it is not a shortcut around immigration, citizenship or benefit rules that have their own separate requirements. Because eligibility turns on facts about the past — how long, and how openly, the person was treated as your child — anyone considering it should gather that proof early and confirm the current administrative steps before starting.