I can see myself using the word ‘disestablishmentarianism’ in a sentence. For instance:

The longest word in the English language is disestablishmentarianism.

My kids find the word disestablishmentarianism funny.

Oh, don’t be such a disestablishmentarian for the sake of being a disestablishmentarian.

Try to say disestablishmentarianism five times quickly (insert pub noise in the background).

I cannot see myself using the word ‘septuagenarian’. Definition:

1. Being between the age of 70 and 79, inclusive. In one’s eighth decade.
2. Of or related to a septuagenarian.

I just can not fathom the need to use this word in a sentence, even though Jim Collins worked it into his book. I will just say ‘old’.