No doubt you must have seen two people of the same age but different physical outlook, how about a set of twins with different physical outlook, have you ever observed and compared your age mates with respect to your physical outlook?
If you have been a good observant in your community, place of work, place of studies, and even in entertainment industry among others, you would have observed it many times that some people appeared older than their actual age, while others with the same age may look considerably younger, thus this is nothing but the differences in the acceleration of biological age relative to their chronological age.
Source from Flickr
Ronaldo who was born on February 5th, 1985 (age 36 years), looks younger when compared with his chronological age mate Rooney who was also born on October 24th, 1985 (age 36 years), despite Ronaldo, being eight (8) months and 19 days older
Having established this fact, and before considering the factors responsible for such differences, it is enough to state that aging is irreversible, it occurs in the entire life cycle of every creature and involves every cell,molecule, and organ in the body. In human being, aging denotes the accumulation of changes in a human being over time, encompassing physical, psychological, and social change.
Chronological Age
Your chronological age can be referred to the total amount of time that has passed since your child birth till now. It usually used to denotes your age from your birth to the current date in terms of years, months, and days as mentioned for Ronaldo (February 5th, 1985, age 36 years) and Ronney (October 24th, 1985, age 36 years).
Biological Age
Your biological age can be referred to how old your body appear, especially your facial outlook, your voice and physical posture are usually what people consider when judging your biological age, may be that is the reason why many people want baby face, sonorous voice with slim fit posture.
Factors Responsible For Biological Age Difference
Genetics factor
No doubt, genetics plays a lot towards our biological age differences, it can be used to determine the biological age as being differ from chronological age, a typical example is the aging clock developed by researchers at the University of Cologne, it has the potential to reads the expected biological age using a sample of its gene. Some are also opined that to know how your biological age might look like, you only need to consider your old parents, since skin elasticity and many more are inheritable from one's parents. Melanin which also protects against photoaging (skin damage due to prolong exposure to the sun’s rays) has been observed to be more in people with dark skin when compared with people with white skin, thereby making dark skin under the same condition with white skin ages slower
Too Much Exposure to Sun's ray
We all need sunlight because it is a good source of vitamin D, however, too much of everything is not good, likewise too much of sunlight may be harmful and cause skin damage as a result of its long accumulation in the body, thereby making the skin looks prematurely aged. It can cause dark spots, wrinkles, freckles, and other issues as well as pre-cancerous and cancerous lesions.
Not Getting Enough Sleep
Generally, having a sound sleep returns a lot of benefits to the body system, like repairing of brain and the whole body system, protection against illness and chronic disease, protection against an uneven pigmentation and an increased elasticity in one's skin, protection against an increase in fine lines which may hinder one's biological age, that is making one to look older, and lack of sound sleep may generate all the aforementioned diseases.
Smoking
One of the effects of smoking is that it reduces one's immune system, it is also mentioned 
Stress
Research has proven the fact that chronic and undue stress can cause premature wrinkles, grey hair. Cortisol, the most prevalent stress hormone, can break down the collagen in one's skin. Aging is pertinent to those who constantly undergoing stress either emotional or physical stress, they both have the potential to affect one's anti-aging routine.
Taking Too Much of Sugar
Nowadays, sugar has become an essential ingredients in almost everything we eat, especially soft drinks, biscuits, cake teat, butter, e.t.c. Research has shown that too much of sugar in one's diet can cause dark circles, wrinkles, dehydrate skin and can fast track the aging process.
Inadequate Water Intake
Water is an important food in our body system and inadequate of water may shut down the whole body system. Inadequate of water is harmful and cause series of damage to one's skin. Dehydration in the skin weakens its defense and creates skin sensitivity, irritation, and premature aging.
Exercise
Yes, daily exercise is also a factor that matters towards ensuring proper biological age.
Final Thought!
A lot of things are responsible for biological age as being mentioned here, we all prefer baby face, and nice physical outlook but to what extent do we care for all the aforementioned factors that are responsible for biological age difference, to see a change in our biological age, we must take necessary measures towards the aforementioned factors, especially the ones we can control aside the genetic factor which is fixed by nature.
Thanks for stopping by always, your contribution is well appreciated and I also wish you a nice and enjoyable weekend and holiday ahead.