Update: If you read this post, I recommend you also read this one. It clarifies a few things I got wrong or expressed poorly here.
Whether or not ageing ought to be considered a disease is still matter of controversy, both among experts and laypeople. Particularly, the latter tend to turn up their noses at the thought of ageing being pathological and not ‘normal’, especially if they’re outside the life-extension/rejuvenation community. Clearly, they ignore the fact that ‘normal’ and ‘pathological’ aren’t mutually exclusive at all. It’s perfectly normal to suffer from hearing loss in old age; notwithstanding, it is out of the question that hearing loss is a pathology and we have developed several ways to make up for it. It presently can’t be cured, because like all age-related diseases, it can only get worse as long as the age-related damage that causes it keeps accumulating.
In my humble opinion of quasi-layperson (I’m nowhere near being an expert, but I do think I know about ageing more than your average Joe), whether or not ageing is a disease is merely a matter of semantics, depending largely on what we want to label as ‘ageing’—not to mention how we define ‘disease’.
If we say that ‘ageing’ is the set of age-related pathologies that affect a given person, then ageing isn’t a disease any more than a box of crayons is itself a crayon. Nonetheless, if you have a box of crayons then you have a bunch of crayons; if you have ageing as we defined it, then you have a bunch of diseases, and the grand total of your ailments doesn’t change whether you consider ageing as a disease as well or not. Quite frankly, I’d pick the box of crayons over ageing any time.
We could define ‘ageing’ differently. We could define it as the damage accumulation processes that eventually give rise to the pathologies of old age. This is a much more sensible definition, because it emphasises the fact ageing is a process that happens gradually over time, starting on day 1. You don’t ‘get’ ageing late in life; you were born with it. When ageing is in its early stages, for example in your 20s or 30s, you can’t really call its effects a ‘disease’ any more than you can call a spec of dust a ‘dust cloud’; when you’re 20, you’re no more ‘sick’ with ageing than a table with a single dust spec on its surface is ‘dusty’. However, during later stages of ageing pathologies are the norm, and the progression of the ageing process exacerbates them further. According to this definition, ageing is still not a disease, but its the cause of many diseases, in pretty much the same way a virus is not the disease it causes: Rhinoviruses are not the common cold; they merely cause it. (This is where the analogy stops. All ageing and viruses have in common is that they both cause diseases. Ageing is certainly not an infectious pathogen!) Notice that, even though HIV, for example, is not itself a disease, we can all agree that we should get rid of it because it causes a horrible disease, namely AIDS. For the same reason, even if ageing did not fit our definition of disease, it is clear that it causes horrible diseases; this should be enough to stop bickering over semantics and just focus on getting rid of ageing already.
We could also think of ageing as an ‘über disease’: A disease whose symptoms are diseases themselves; a ‘disease of diseases’. This is more along the lines of what Aubrey de Grey calls ageing, and he’s not wrong, because what we currently see happening in old age is essentially the sum of different age-related pathologies all happening at more or less the same time.
If you ask me, even without going into the details of the biology of ageing, I’d say that, strictly speaking, it’s probably not a disease (some say it’s neither a disease, nor a non-disease), but it obviously causes crippling pathologies; however, if classifying ageing as a disease may help us get sooner to a world free of age-related diseases, I’m definitely in favour of doing it. I’ll gladly discuss the semantics of the matter after the diseases of old age will no longer be a problem. (*)
(*) Please, do have a look at my first comment below for a further clarification of my stand on the matter.