That seems... extremely specific to me. That's the only way to learn something? Or the only way to learn first-level abilities, but they can learn higher-level ones?
Are you familiar with the old Guild Apprenticeship system? It's still sort of used in some trades. A master craftsmen accepts children (generally 12-14) as apprentices. They spend 7 years learning their craft (and serving as cheap labour), and then they become Journeymen, who are junior members of the guild with limited rights to ply their trade, which they do, until they hone their skills sufficiently to test for the status of Master, whereupon they create a masterpiece to be judged by the senior tradesmen, and if they judge it a worthy masterpiece he become a master with full guild privileges including the right to take apprentices.
A 1st level character is basically a newly minted journeyman. He has a solid base of skills, and is now engaged in refining them (by earning XP) until he becomes a Master. Which I would generally put around 5th level.
Historical knights used pretty mush the same system, except the apprentices were called squires, and they didn't create a masterpiece, they earned their spurs.
A 1st level wizard is not 12 yr old Harry Potter trembling on the steps of Hogwarts, he is 19 year old Harry who has been studying magic for 7 years and is ready to show up for his first day of work as an Aurour.
Or to put it in modern educational terms, once you get to University you spend 4 years getting a bachelors degree which lays the groundwork for your future work. (Getting to 1st level.) You then pursue a Masters Degree where you you have much more control of your agenda, because you are assumed to understand the basics of your field, and are instead refining your speciality. (low level adventure) At the Doctoral level you're more working with the professors as a peer than under them, you're probably teaching classes yourself. (Upper level adventurer)
In the Martial Arts you spend 3-5 years earning the right to test for a black belt, at which point you're considered to be a competent practitioner, which plenty of room to grow, but capable of teaching others while you improve your own skills. Note that the Monk class, which includes skills and magical training is much more involved and intense then mere sport martial arts training.
Do you see the theme here? Professional competence, both historically and in the modern world, is the result of years of effort spent building a base of skills. Once acquired these skills form a foundation that is built upon to achieve true expertise. True expertise btw is usually reckoned to be the product of 10 years of effort, or about 10,000 hours of training.
Someone who dabbles in a skill does not have PC class levels, they have commoner levels, if they are lucky. Your 1st level PCs have spent years learning their trade (possibly decades for elves.)
Now, PCs are assumed to be extraordinary even by the standards of their profession. If a PC want's to multiclass, and I was allowing multiclassing, I would work with them to find a trainer, or McGuffin such that they can get a training montage done in some reasonable span of time, rather than 7-12 years (Records indicate it took 12 years to become a Bard back in the day. Without being a full-caster.) But I would not normally allow someone to switch classes without some kind of in world justification. Which would be spelled out in the campaign expectations before play started, I don't surprise people with that kind of stuff, since opinions differ.
