I will try to answer this question. I think lifelong learning isn't theory, but a concept. The first step we have to clarify what the different between "theory" and "concept" for determine lifelong learning is.
"A theory explains why and how something happens, based on research or scientific evidence. "A concept is a fundamental idea or framework used to describe or understand something without necessarily explaining the reasoning behind it."
Back to the question, so i think lifelong learning is a concept.
In Islam (my religion) we know this (lifelong learning) as hadith sourced from prophet Muhammad PBUH. "Uthlubul 'ilma minal mahdi ilal lahdi" meaning "Seek knowledge from the cradle to the grave. This hadith indicates that the pursuit of knowledge has no age or time limits, making it an obligation that must be carried out throughout life, from birth until death."
You can add too, search paper "The Father of Indonesian Education: Ki Hadjar Dewantara and His Educational Philosophy" as a reference.
I hope my answer helps Irery Lizbeth Melchor-Durán
You raised an important issue. Please let me argue that Lifelong Learning is an umbrella type of research domain:
Thwe, W.P., Kálmán, A. Lifelong Learning in the Educational Setting: A Systematic Literature Review. Asia-Pacific Edu Res 33, 407–417 (2024). https://doi.org/10.1007/s40299-023-00738-w, Quote: "Our review findings showed that theoretical papers, such as reports, policies, and concepts of lifelong learning, are generally much more extensive than empirical studies."
Open access:
Article Lifelong Learning in the Educational Setting: A Systematic L...
2) A case study:
Håkansson Lindqvist, M., Mozelius, P., Jaldemark, J., & Cleveland Innes, M. (2023). Higher education transformation towards lifelong learning in a digital era – a scoping literature review. International Journal of Lifelong Education, 43(1), 24–38. https://doi.org/10.1080/02601370.2023.2279047, Open access:
Article Higher education transformation towards lifelong learning in...
May I argue that lifelong learning is a systemic concept including theories and empirical studies in context to be defined in different settings.