Mushrooms are basically food, so will not have an adverse effect on young animals or seniors.
A study using oyster mushrooms in 8 week old puppies found immune system benefits without any evidence of adverse effects after feeding the oyster mushroom extract for 4 weeks to these young, immunocompromised puppies.
- Haladová E, Mojžišová J, Smrčo P, Ondrejková A, Vojtek B, Prokeš M, Petrovová E. Immunomodulatory effect of glucan on specific and nonspecific immunity after vaccination in puppies. Acta Vet Hung. 2011 Mar;59(1):77-86. doi: 10.1556/AVet.59.2011.1.7. PMID: 21354943.