One necessity - or card that plays an important part - of Philosophy is that of logic. Without the path of logic from A to B or to aid the formation of argument structures, Philosophy is rather stranded.
Take the example 'if I am young today, then I will be young tomorrow; if I am young tomorrow, I will be young the day after that - so I will be young in 80 years' time,' a statement which, to me, is nonsense. Let's take it phrase by phrase.
At first there is some logic there: if X is the same one day to the next, in 80 years’ time, it will be still be X. However, X is not equitable to years. I am able to say ‘I am seventeen now, I will be seventeen tomorrow,’ because it will not have been my birthday between those two days, but in a year’s time, I will no longer be seventeen.
We are able to use the same critique of reason for the argument of the ‘young’. It is not correct to say that an object that, by definition, changes over time (we, as humans, are bound to age), is fixed unless there is another predicate to contradict the former. This is the sort of logic that philosophers face every day: meta-logic, epistemology, language analytics.
You see, the word ‘young’ implies the opposite of being ‘old’, something we equip with age. There does indeed become a point in the human life as we know when someone stops being ‘young’. Time does not affect the word, but its meaning. These sorts of ‘necessary meaning’ words are in the world all around us; ‘the bachelor is an unmarried man’ is a sentence that tells us nothing, because we already know that the subject of the sentence is an unmarried man – the word ‘bachelor’ implies that. Using this logic, the word ‘young’ implies lack of total age.
There is a paradox in the statement that defies existing knowledge. If someone whom I consider ‘old’ (of 80 years + age) is ‘young’ forever, what defines this youth if there is nothing to contrast it with? 'Youth' is a predicate of being a certain age; someone who is young must be young, otherwise the word 'young' would possess no meaning, so this sentence becomes non-sensical.
Against that statement we could also argue what we empirically know. The sentence must go against systems of age and numbers that we have grown up with. Perhaps one could go so far as to say that we know these pieces of information innately. It depends on a person’s opinion of what it is to be young. Not young at heart, as such, but what they consider the guideline to be. If one were to consider themselves old – in that they possessed more years than another - aging one day, eighty years would make no difference to the reflection of being old in comparison with another. Too, if the sentence subject could provide someone to measure ‘youth’ against, they might have a case. However, I don’t believe we can use this particular logic the other way.
Thus, no one can be young forever, if they change age over the course of 80 years. Were we to change the 80 years into something without measure, perhaps the statement might become meaningful.
 description of assets.
 See Hume’s Fork or Ayer’s Verification Principle