What is the oldest letter in the alphabet and what is the most recent?

The oldest letter in our alphabet is o, which has existed in its present form for more than 3,000 years!
http://www.bigsiteofamazingfacts.com/why-is-z-the-last-letter-in-our-alphabet

J is the last addition to our modern alphabet. Scholars kept it from dictionaries until the late 19th century. It is descended from I, and in medieval times was used as a consonantal form of I. J is one of only two capitol letters with a descender (the other being Q).
https://adebayothevoice.com/qa/what-is-the-27th-letter.html

More about O:
O and capitol I make up the two basic forms of all letters in our modern alphabet. O is unique also in that its forms are basically the same between upper and lower-case letters. The letter descended from the the Semitic “ayin” which meant the word “eye” in Phoenician. The Greeks adapted O from the Phoenicians and used it as the vowel “omnicron” in their early alphabet. In later Greek a break occured between the two Greek variant alphabets — the Chalcidean (west) and Ionion (east). Chalcidian became the basis of the Roman alphabet, wheras the eastern, or Ionion, became classic Greek. The Ionian alphabet replaced the O for omega, placing it at the end of their alphabet.
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