Commentary:
That's not a slip of the pen: Dalris is a bard, the AD&D character class. Strap in while I drop some four-decades-old knowledge.
Page 117 of the AD&D Players Handbook presents the bard class.
To even qualify for the bard class you needed MINIMUMS of Strength 15, Dexterity 15, Constitution 10, Intelligence 12, Wisdom 15, and Charisma 15.
There is no way you rolled those ability scores on 3d6 in order unless you cheated like a madman.
But even with “I got amazingly lucky (wink)” ability scores, the bard wasn't a normal class that you enter as a fresh 1st level character. Oh no. Not even close. The bard was effectively the first Prestige Class or Paragon Path: you had to work your way up to it.
First you began play as a fighter until at least level 5.
Then you switched to thief ("rogue") until at least level 5 of that class.
You were at that point a level 10 character overall.
THEN you took training with a druid. But that actually made you a bard. As a bard you got druid spells and bard abilities. (Mostly the same ones you get today: inspire your allies; counteract sound-based magic; know a little about a lot of things.)
You could thereafter advance in character level as a bard, up to level 23 where you capped out. (Why level 23? Because that's where the bard's progression table reached a nice round 3,000,000 XP.)
Let's get back to Dalris.
To be described by the book as a bard, even a bard-in-training (like Carr is a magic-user in training), Dalris would previously have to be AT LEAST a level 5 fighter AND a level 5 thief. Level 10 overall.
She would then need a druid to teach her druid-y stuff. Conveniently her father Perth could do that.
So at that point the grizzled Swashbuckler (5th level fighter title) / Robber (5th level thief title) / Rhymer (1st level bard title)... also decided to study magic-user magic with Landor for some reason? For goodness sake, woman, WHY? It's not like you needed to fritter away your XP on yet another character class!
And let's ask again: how old is Dalris?
Suppose she began her fighter-ing at age 16, the absolute youngest age a 1st level human fighter could be according to the AD&D Dungeon Master's Guide (page 12). She's a prodigy when it comes to fighter-ing and reaches level 5 in just 1 year.
She goes off to be a thief. She's also a prodigy at thief-ing and reaches level 5 in that class in just 1 more year. She's now 18, one year YOUNGER than the minimum 1st level human thief age according to the DMG, but we’ll allow it.
Dalris then studies druid magic with her dad and due to nepotism and favoritism, passes her bard classes in a week (hey, it worked for Carr with magic-user classes!), so she's still only 18.
Then for some reason Dalris studies magic-user magic with Landor. Maybe she doesn't get very far or maybe Landor dies soon after she starts, so maybe only another week has passed. Dalris is still just over 18 years old.
But that was FIFTEEN YEARS AGO. So she's 33 years old now. And this is assuming the most favorable possible advancement rates imaginable.
Although I will note that per the AD&D DMG, the minimum age of a 1st-level human magic-user is 26, and Carr is only 15, so we're not playing by the rulebook when it comes to character ages in this series.
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An addition with the benefit of hindsight that will be seamless to readers, but was actually written several days after the above.
One of my old-timer friends pointed out that it didn't take anywhere near two years to reach level 10 or 11 in an AD&D character class. With regular play, assuming your character didn't die, and pretty much all you did was enter the dungeon, kill monsters, grab loot, rest in town, repeat -- you could easily reach level 5 in a couple of weeks of game time and level 10 within several months.
Thus, plausibly Dalris could start her bard training six months after she began her adventuring career. Which could've been just a bit before Carr.
That just leaves her claim to have studied magic with Landor. As my friend pointed out, maybe Dalris exaggerates this claim the same way she exaggerates her claim to be a direct descendant of Bhukodian / Kandian royalty. "Studied magic with Landor" might mean that baby Dalris drooled on the pages of Landor's spellbook while she cooed on his lap, or maybe toddler Dalris looked at pictures while Landor was preparing his spells for the day.
Or, heck, maybe Dalris was an incredible prodigy like
John Stuart Mill, who was taught Greek at age 3! So she could have studied magic at a very young age, then Landor died, then some years later she spent six months getting ready to be a bard, then she just got her bard certificate last week. Which would make her maybe no more than 18 years old "now".
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As for the tower climb: the specifically described window means that anyone could use that window of Landor's room to bypass the murder door to gain access to the ultimate magical power stored within.
In a gamebook path we didn't take, Beldon uses the Fly spell. Beldon didn't need to wait fifteen years after Landon’s death for Carr to show up to get into Landor's quarters. Beldon could've entered Landor's quarters fifteen MINUTES after Landor’s death.
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We'd better stop asking questions before we fail a bunch of Sanity checks and have our Wisdoms reduced to 3 like Carr.