Understanding the initial formation and development of lichens is crucial for elucidating the mechanisms behind the formation of complex lichen thalli and their maintenance in long-term symbioses. These symbiotic relationships provide significant ecological advantages for both partners, expanding their ecological niches and allowing them, in many cases, to overcome extreme environmental conditions. The correct development of thalli likely relies on the selection of suitable photobionts from the environment. In this study, we focused on the impact of lichen age on the overall diversity of photobiont partners and examined how mycobiont preference toward their symbionts changes at different developmental stages. Using the lichen Protoparmeliopsis muralis as a model organism, we observed a strong correlation between the diversity of photobionts and lichen age, confirmed by both molecular data and morphological observations. Our findings indicate greater photobiont diversity in older thalli, suggesting that lichens retain the majority of algae they collect throughout their lifespan, potentially as an adaptation to changing environmental conditions. Additionally, we found that some lichen samples contained only low levels of Trebouxia algae, indicating that P. muralis does not consistently rely on this typical partner and that local environmental conditions may significantly influence its symbiotic composition. Keywords: DNA metabarcoding, lichen ontogeny, photobiont diversity, Protoparmeliopsis muralis, selectivity, Trebouxia.