I noticed "senissimus" appears on Wiktionary, but it's not obvious if there are any other attestations of this word. How does one investigate whether this is a spurious Wiktionary page, or is indeed a Latin word once in use? The same goes for juvenissimus.
Latin is a heavily inflected language, and only very specific forms are found in dictionaries. To see whether a form would be legitimate, it is best to understand how it is formed. Even if a specific form was not attested at all, we can be perfectly sure that it is a fully legitimate Latin word — by analogy to attested forms of a word and inflection of other words. This does require some expertise, and it is not wrong to ask for help. (That's what this site is for!)
In this specific instance the question is whether superlatives of senex are really senissim-. The answer from a classical point of view is "not really". The best way to check whether something was actually used is to consult a text corpus. They all have their limitations, but they do give good hints at least. There is only one hit for senissim- but 27 for veterrim- and 105 for vetustissim-.
If you consult the irregular comparative and superlative section of a Latin grammar, it is likely to mention that superlative forms of senex tend to be replaced by something based on vetus. The comparative senior is common, and so is iunior. I urge you to try a corpus tool to see how common iuvenissim- is.
The Wiktionary entry you linked to has a reference "senissimus in Charles du Fresne du Cange’s Glossarium Mediæ et Infimæ Latinitatis (augmented edition, 1883–1887)", which in turn links to http://ducange.enc.sorbonne.fr/SENISSIMUS :
SENISSIMUS, Valde senex. Vita S. Landoaldi tom. 3. Mart. pag. 37 :
Huic repositioni interfuit quidam Frangerus, homo nostra ætate Senissimus. Testes Senissimi,
in vet. Notitia tom. 2. Capitular. Baluzii col. 823.