Can the preparation be used? Duolife Day & Night at a 13-year-old's?

There are no data to suggest the safety and efficacy of Duolife Day & Night in children and adolescents.
The manufacturer gives a dosage intended for adults only.

In the case of lack of appetite, decreased immunity and vitamin deficiencies in a child, you can use products designed for children and teenagers. These include products available in the offer of online and stationary pharmacies, such as:

https://www.gdziepolek.pl/produkty/11333/bioaron-system-syrop/apteki

https://www.gdziepolek.pl/produkty/82315/vibovit-uczen-tabletki-do-ssania-gryzienia-zucia/apteki

https://www.gdziepolek.pl/produkty/50726/multi-sanostol-syrop/apteki

https://www.gdziepolek.pl/produkty/30408/floradix-zelazo-i-witaminy-plyn/apteki

Choosing the offer of an online pharmacy allows you to compare prices and order the preparation with home delivery or personal collection at the selected pharmacy.

In cities such as Warsaw, Wrocław, Katowice, Kraków, Lublin, Gdańsk, Łódź, Szczecin, Białystok, Poznań and Chorzów, products can be ordered with instant delivery in as little as 1 hour thanks to the Wolt Drive service.

- check if the service is available in your location:

https://www.gdziepolek.pl/koszyk/szybka-dostawa?src=zf

If the use of over-the-counter preparations is not enough for you, I encourage you to consult your child's health with a doctor. You can use teleconsultation, e.g. via the Gdziepolek.pl portal:

https://www.gdziepolek.pl/telekonsultacja/z-pediatra

I encourage you to read additional materials that may be useful:

https://www.gdziepolek.pl/artykuly/wzmacnianie-odpornosci

https://www.gdziepolek.pl/kategorie/witaminy-i-suplementy-dla-dzieci