A long time before we realise it, our bodies age inside. We may have no visible signs or symptoms to show that this is happening, but around the age of 20 the battle begins. Or the decay, to use that awful word, but it does describe the way we evolve physically because once we have lived for a couple of decades the functions of our body begin to diminish.
We will still have a few years of ignorance about what is happening, because the deterioration is very slow at first, but by the time we realise what is going on the loss of function which began in the prime of life is already 80 per cent complete. In other words, when it can no longer be hidden.
"Although certain important medical indicators in the ageing process, such as hormone levels, start to drop from the age of 25 after having reached their peak, it isn't until 40 or 45 that we start to notice the consequences," says José Serres, president of the Spanish Society of Anti-ageing and Longevity Medicine (SEMAL), who is a renowned expert on the subject in Spain.
What do people normally notice first? Basically things that aren't like they used to be, he says: worse digestion, more fatigue after effort, for example. These are normal signs and nothing to be concerned about, but even people who boast that they never have to see a doctor should have check-ups from time to time, to maintain good health and prevent certain ailments.
How often should those check-ups take place? Fidel Fuentes, the head of A&E at the IMQ clinics, says there is a simple way of remembering: "every three years is reasonable for people under 40, every two years for those aged between 40 and 50 and every year from 50 onwards," he says.
To be more specific, here is a guide to the 'MOTs' which are recommended for each age group.
Remember that we are talking about healthy people, so does someone in their twenties or thirties really need check-ups?
"If anyone is worried about something, at any age, they should see a doctor but not panic about it," says Dr Fuentes. For those in their twenties, a blood test is a good idea every three or four years to check that the defences (leukocytes) are OK, sugar levels, iron and cholesterol, (which because of our poor diet is occurring at an increasingly earlier age).
Will that be enough? No. At this age, the greatest risks to health are athletic exertion and sexually transmitted diseases. For obvious reasons, the body asks for and allows both. So, sporty types are recommended to do stress tests (once a year if they do high-intensity exercise) and those who are sexually active are advised to check for STDs.
"Of course, we also have to pay attention to early detection of cervical cancer in women aged 25 to 35 with a smear test every three years, and from that age a test for HPV (human papilloma) to the age of 69," says Francisco José Sáez, the head of the Chronicity group at the Spanish Society of General and Family Doctors (SEMG).
Dental check-ups should be done annually, and eye tests every two or three years as long as no problems have arisen. All these recommendations are for people in their twenties.
Do things change much once we're in our thirties? The frequency of blood tests need not vary if no specific problems are discovered, but it is important to start paying attention to the skin and any moles or marks that start to occur and don't look good.
Everything is probably going to go smoothly in this decade, as long as we look after ourselves a little and have no family history of serious illnesses. Of course, the check-ups should continue as before, but the blood tests should be done more frequently, every two years or three at the most.
Women should keep an eye on their menstruation because if it stops it could be an ovarian problem and not an early menopause. Anything else? Yes, eyesight checks! Men and women normally note a loss of sight around the age of 45: nearly everyone suffers from eyestrain and needs glasses, so annual eye tests are necessary.
This is the turning point, when we should start looking after ourselves better. "The prevalence of tumors increases," says Dr Fuentes, so there is more screening for breast and colon cancer on offer and we should make sure we don't miss it. Smokers need to check for signs of lung cancer, and women still need to continue with cervical smear tests.
High blood pressure, coronary problems and cholesterol must all be controlled because otherwise they can lead to serious cardiovascular problems.
For men, prostate checks from the age of around 55 are a must, especially if they notice changes in erection or urination.
In our fifties, we also start to lose hearing and notice that we can't always make out words if there is noise in the background... so if that is the case, it's best to get it checked.
"At this age many people who have never bothered with check-ups discover that they have high cholesterol and triglycerides, they are overweight (sometimes they think it's no big deal, but it is starting to be a problem) and their blood pressure is high," says Dr Fuentes.
Well, this is the time to start taking these things seriously.
Once we reach our sixties it is advisable to get older with as high a quality of life as possible, so check eyesight, hearing, dental health... all in addition to the check-ups we have been accustomed to having in previous decades.
"There is no need to get obsessed over the check-ups, but if someone is healthy and has one and doctors spot something that isn't quite right then they can do something about it by changing lifestyle habits or redirect things that aren't being done well," says Dr Fuentes.
And then, of course, if we are taking medication for a chronic condition, we need to check occasionally to make sure that it isn't having any negative effect on any of our body's functions.
Some groups of people, because of their family or personal medical history, need more check-ups than the population in general
Lung cancer is a sword of Damocles hanging over them, so from their thirties onwards they should be checked regularly for this.
The normal monitoring by the health service should be sufficient in most cases.
From the age of 16 athletes who do high-intensity sports should have stress tests every two or three years until they are 35, when the tests should be carried out annually. Anyone who starts practising sport at that age and has never had a stress test should also do so.
The risk of diabetes, high blood pressure or vascular illness needs to be controlled. And they should change their diet and lifestyle habits as soon as possible.