Meh. Totally different balancing techniques between editions.
5e
- increase hit points on all monsters by 100-300%.
- spell power scaling only happens with up-casting
- Fewer spell slots after Class Level 8
- Never get more than a single 8th or 9th level spell slots
- Repeat saves for spells
- Concentration limits number of active spells with duration
- Legendary Resistance is common on "boss fights"
1E
- Spell slot assignment in advance
- Spell recovery is slow
- Spells are easily disrupted during casting
- Saves are easier to make
- No freely cast cantrips or rituals
So in-game historians might say "Magic of the first recorded age was devastatingly powerful. A caster capable of Teleport was able to kill trolls with a single fireball and even dragons could fall to two or maybe three castings of the appropriate spell. However they might not have that spell and even if they had it, a small child could tug on their robes to interrupt the casting. Many casters took to flying while invisible and surrounded by protective barriers, to avoid that fate, though they had to plan that much farther in advance. And once they cast those spells, it might take them an hour just to recover one spell and a full complement of spells might take days."
"By the 5th age, creatures had become tougher. A fireball alone simply cannot kill a troll, it requires a Meteor swarm or the like. A caster capable of teleport might exhaust their entire array of spells against an adult dragon before maybe, possibly, slaying it, though a majority of their most effective spells will likely be resisted. They will have far fewer spells of great power; gone are the days when a titled magic user could even hope of casting a dozen fireballs. Conversely, these fewer, weaker spells do not require 'pre-casting' and can be recovered quickly, no more than one night's rest and some light meditation. This also means casters do not pre-select the exact number of each spell they have available so they are more likely to have a useful, if not awe inspiring, spell available. Furthermore, casters can be pummeled by multiple enemies on the deck of a storm tossed ship and still effectively cast spells. Which may happen as it is no longer possible for a caster to keep a flight spell and invisibility active, let alone a layered defense. Casters have an array of simple combat magics and utility spells that may be used without using their reserves, though some do require as much or more time to cast as earlier ages would need to recover them."
"Would you prefer to live in a time of powerful spells that require great planning and care to use effectively, or when casters had weak magic they could use reliably? Let us look now to the 6th age...."