I'm struggling to come up with a reason as to why our character has the physic of an old man
I think a few people give good points as to why. Thematically I would think it has a lot to do with this notion of a "mage" or "tactician". Maybe the awakening community was excited robin's in the game, but I'm pretty sure more Fire Emblem fans of longer pedigree have been begging for a mage
forever, and Sakurai gave us a mage, tried and true in every design facet, he did an amazing job.
As a mage they aren't front line fighters, in some games they have one less movement than other foot classes usually to emphasise this.
They're an option rich character, being able to hit freely at both 1 or 2 range, being able to counter other ranged characters up close, while countering melee fighters from afar. But whenever they're in a position to be counter attacked, they're often very squishie, and cannot afford that to happen. You have to appreciate their strategic niche.
Being the game tactician, well that often implies a strategist who's role isn't on the battlefield (a common element of all other games before awakening). Combine the two and you have this idea of having to be careful with your movements, as strategically you have "all the tools" from any given position that you have chosen to be in. For you the player, or in some cases smart AI, you always deal with the mage first and
approach them, as their capabilities to decimate an army unchecked is mostly unparalleled considering generally low resistance.
How is a mage a rush down character? I'm sure we've all had mages in fire emblem games that were gods and could solo armies, and in the case of a
wind mage, well some are canonically pretty frikken fast (my boy Levin #TheOriginal [not a thunder mage]), they kept up with Pegasus Knights, but I digress.
Basically, this character has a lot of range, a lot of projectiles, and considering this, pretty alright frame data with great hitboxes, albeit pretty harshly limited to not be able to rush down/punish (hint: slow grab [the main punishment tool in every game], slow run), the character isn't designed to do that. You press your advantages hard, and you cannot be 'ignored', pretty great balance. Realistically, everything Zelda could've ever wanted.