Short answer. Fifteen, with narrow exceptions, and nobody under eighteen may be put on hazardous work. The Labor Code article people quote is superseded: Republic Act No. 9231 (2003), amending Republic Act No. 7610, governs child labour now and is materially stricter than the old text.
What the law says
No child below fifteen (15) years of age shall be employed, except when he works directly under the sole responsibility of his parents or guardian, and his employment does not in any way interfere with his schooling.
Labor Code, Article 139 — Minimum Employable Age. Read the full provision →
The floor, and the law that sets it
Article 139 of the Labor Code still reads No child below fifteen (15) years of age shall be employed, except when he works directly under the sole responsibility of his parents or guardian, and his employment does not in any way interfere with his schooling. The age of fifteen is right. The rest is not the current rule. Republic Act No. 9231 (2003), which amended Republic Act No. 7610, now governs child labour, and it is stricter than the article above: it restricts the permitted undertakings, requires a work permit from the labour department, caps working hours, and requires schooling to continue. Treat the Code text as background.
Fifteen to eighteen is not unrestricted
The Code article said a person between fifteen and eighteen may be employed for such hours and periods of the day as the labour department determines by regulation. Under Republic Act No. 9231 that supervision is much tighter than a bare reference to regulations suggests, with limits on daily and weekly hours and on the times of day a young worker may be made to work, and with schooling protected. An employer cannot treat a sixteen-year-old as an ordinary adult employee on the roster. The specific caps are in that Act and its rules, so check them there rather than assuming the old article covers it.
Under eighteen and hazardous work
One line of the old article has not softened: no person below eighteen may be employed in an undertaking that is hazardous or deleterious in nature. The current framework keeps that bar and reinforces it, and the labour department maintains the list of what counts as hazardous. This is an absolute limit rather than something a parent's consent, a training label or an apprenticeship arrangement can waive. If a workplace is on that list, the age of the worker settles the matter regardless of how willing the young person or the family is.
The exception is narrower than it sounds
People read the exception for work under a parent's responsibility as a general licence for family businesses. It is not, and under Republic Act No. 9231 it is narrower still: the permitted situations are limited, a permit from the labour department is required, the hours are capped and the child's schooling must continue. Employing a child under fifteen without meeting those conditions is child labour, not a family arrangement. This page gives general legal information about which law now applies; the detailed conditions must be read in the Act itself, and no page can clear a particular arrangement in advance.