老化とは、生物学的には時間の経過とともに生物の個体に起こる変化。その中でも特に生物が死に至るまでの間に起こる機能低下やその過程を指す。
学術分野では発生、成熟、老化などを含めた生物の時間変化すべてを含む言葉として「老化」を用いる。例えば、樹木の葉が加齢と共に黄色くなってやがて落ちるのも、同じく樹木が発芽してからの生長するに従って、挿し木時の発根や成長程度が悪くなるのも、動物が生まれてから時間が経つに従って、活動性が低くなりやがて死に至るのも「老化」、と表現されるが、その起こっている事象は全く別であると考えられており、混同すべきではない。

Aging or ageing is the process of becoming older. The term refers especially to human beings, many animals, and fungi, whereas for example bacteria, perennial plants and some simple animals are potentially biologically immortal. In the broader sense, aging can refer to single cells within an organism which have ceased dividing (cellular senescence) or to the population of a species (population ageing).
In humans, aging represents the accumulation of changes in a human being over time, encompassing physical, psychological, and social changes. Reaction time, for example, may slow with age, while knowledge of world events and wisdom may expand. Aging is among the greatest known risk factors for most human diseases: of the roughly 150,000 people who die each day across the globe, about two thirds die from age-related causes.
The causes of aging are uncertain; current theories are assigned to the damage concept, whereby the accumulation of damage (such as DNA oxidation) may cause biological systems to fail, or to the programmed aging concept, whereby internal processes (such as DNA methylation) may cause aging. Programmed aging should not be confused with programmed cell death (apoptosis).
In 1934, it was discovered that calorie restriction can extend lifespan by 50% in rats and this has motivated research into delaying and preventing aging.