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I would absolutely consider maturation and aging as two separate processes. Maturation can be seen as a physiological process (IE normal) while aging can be thought of as pathological (IE bad). Both progress at different rates for different systems and are enormously influenced by environmental factors.

Sexual development, for example, begins during puberty (~ages 9-11) and lasts for 2-4 years. Increased body fat can cause early puberty while decreased body fat can delay it. Contrast that to brain maturation which begins at birth (or at conception, some may argue) and continues through the age of 25. Around this age is when most people considered humans "fully developed" biologically.

Aging can "begin" at nearly any time depending on many factors including diet, exercise, sleep, stress, etc. In fact, risk of disease and/or death is better predicted by using calculations of biological age (IE how well does your body function) than chronological age.

Muscle loss is often seen in the elderly often thought to begin in middle age and continually progress. However, a strength building exercise program can often halt this loss or even reverse it.

Cognitive decline is often seen as an inescapable feature of aging. However, multiple studies have shown that diet, exercise, sleep, and stress management (IE lifestyle interventions) can reduce risk for Alzheimer's by nearly 60%. In fact, in a clinical trial for lifestyle interventions to prevent dementia, they found that those that adhered to the protocol actually improved cognitive performance over 2 years.

Metabolism is often blamed for weight gain as we age. A large study of over 6000 participants found that there is no change in metabolism from age 20-60. They did find a strong correlation to increased metabolism with increased lean mass (fat-free mass).

That isn't to say we can become immortal by doing push ups and having a kale salad now and again. As we live longer, we accumulate risk for disease. It can be thought of as rolling the dice the over and over again and eventually something will happen. Since aging is inevitable (at least for now), starting good habits young will help make sure that each time those dice are rolled, the risk for disease is as low as possible. This is often called "healthy aging".

Human development - https://www.britannica.com/science/human-development

Body fat and sexual development - https://www.nature.com/articles/nrendo.2011.241

Brain maturation in humans - www.ncbi.nlm.nih.gov/pmc/articles/PMC3621648

Biological age is a better predictor of mortality than chronological age - academic.oup.com/biomedgerontology/article/72/7/877/2629918

Preventing age-related muscle loss - www.ncbi.nlm.nih.gov/pmc/articles/PMC2804956/

Lifestyle interventions prevent AD by up to 60% - lifestylemedicine.org/wp-content/uploads/2022/08/ACLM-Article-Lifestyle-Intervention-Alzheimers.pdf

Lifestyle interventions improve cognition in those at risk for dementia - alz-journals.onlinelibrary.wiley.com/doi/10.1002/alz.12492

Metabolism is stable between 20-60 (editorial) - www.health.harvard.edu/blog/surprising-findings-about-metabolism-and-age-202110082613

Metabolism is stable between 20-60 (scientific article) - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8370708/

Healthy aging via lifestyle changes - www.sciencedirect.com/science/article/pii/S0304416509000208

Generally in development, maturation stops once adult size is reached (at least in most animals - a few grow throughout life). This tends to coincide with sexual maturity i.e. when the animal becomes reproductively active.

But the process can be different for different parts of the body. For example, humans reach maximum number of neurons just before they are born - from then on it's downhill all the way. But brain maturation continues until about age 20 with myelination of axons.