I think both "ald" and "gammal" (or similar) did exist in predecessing language(s), and meant essentially the same. Over time, gammal won the positive, but äldre and äldst prevailed in comparative and superlative.
In German, it's "alt, älter, (am) ältesten", and "gammal" has only survived through the verb "gammeln", which means (for food) "to go bad".
Comparable to "good, better, best" - "bra, bättre, bäst".
@CMShifflett, that reminds me of the time I was reading the Pentamerone on a flight and the child next to me asked me to read for them once it became apparent that I was reading fairy tales. It was a late 19th century edition, and I had to do a remarkable amount of censoring such as change "❤❤❤❤❤" to "dog" and subtly skip two pages of, shall we say, mammarial appreciation.
The mother of the child got to sleep through the flight and was very grateful. I was equally happy that she slept through any censorship mishaps to which I may have subjected her child. :)
The fairy tales I read as a child were in English, but it was a special Older English where a child was never the "oldest" but the "eldest" and didn't just "wear nice clothes" but were "clad in fine [raiment]."
We still have youngsters and elders but "eld" (and "raiment") and "clad" (as the past participle of "to clothe") have largely vanished since the 1800's. These lessons are like a time-tunnel to the past. Thank you Arnauti and all of you who make this possible!
We have similar in English: "good", "better", "best". In standard English there is no "gooder" or "goodest", nor is there a "bet" for "good". Or perhaps it should be "bat" or "boot", as detailed here: http://www.word-detective.com/2010/10/good-better-best/
de yngsta och de äldsta is plural but in the masculine singular you could say den yngste och den äldste. The English sentence is ambiguous, you don't know if it's about one (ok, two) or more people, but in Swedish you have to choose between singular and plural. (the masculine form is optional even for masculine beings).
Actually, Swedish has only one remnant of our old gender system in place - namely, that you can optionally use an -e ending for masculines in the definite singular. So you could say yngste or äldste about a boy, for instance.
Since the base forms are ung and gammal, it would be probably have been ungaste and gamlaste. The latter is a classic example of what toddlers will frequently come up with until they learn the irregular form.
Risto is a common Finnish name, so I'm guessing that Risto is Finnish, in which case his comment makes sense - de is actually pronounced de in Finland Swedish, and dom in Sweden Swedish. I would not be surprised to find that most Swedish speakers in both countries do not know this.