Sr. As notas de Ollivander têm algumas coisas interessantes a dizer sobre isso ... mas primeiro, lembre-se:

1. A criação de varinhas é um segredo da família.
Ollivander é considerado quase indiscutivelmente o melhor.
2. Ele admite abertamente que não entende completamente a tradição da varinha.

The best wands, he believed, had cores of immensely powerful magical substances, which were expertly enclosed in specially selected and complementary wandwoods, the result to be matched to an owner with whom the wand itself felt the most affinity.

...

His methods of locating wand woods and core substances marrying them together and matching them to ideal owners are all jealously guarded secrets that were coveted by rival wandmakers.

(Mr. Ollivander on Pottermore by JK Rowling)

Every single wand is unique and will depend for its character on the particular tree and magical creature from which it derives its materials. Moreover, each wand, from the moment it finds its ideal owner, will begin to learn from and teach its human partner.

...

The following notes on various wand woods should be regarded very much as a starting point, for this is the study of a lifetime, and I continue to learn with every wand I make and match.

(Wand Woods on Pottermore by JK Rowling)

Readers should bear in mind that each wand is the composite of its wood, its core and the experience and nature of its owner; that tendencies of each may counterbalance or outweigh the other; so this can only be a very general overview of an immensely complex subject.

(Wand Cores on Pottermore by JK Rowling)

However, no single aspect of wand composition should be considered in isolation of all the others, and the type of wood, the core and the flexibility may either counterbalance or enhance the attributes of the wand’s length.

...

Wand flexibility or rigidity denotes the degree of adaptability and willingness to change possessed by the wand-and-owner pair - although, again, this factor ought not to be considered separately from the wand wood, core and length, nor of the owner’s life experience and style of magic, all of which will combine to make the wand in question unique.

(Wand Lengths & Flexibility on Pottermore by JK Rowling)

TL; DR Para resumir, enquanto um mago pode substituir sua varinha (como vemos Charlie Weasley) eles não vão "crescer". Cada dono de varinha tem um instrumento único e a varinha aprenderá e crescerá com o mago.

Na grande maioria dos casos, a varinha "cresce" com o assistente.

Depois que um mago pega a varinha pela primeira vez, enquanto a usa, sua varinha aprende e cresce com ela. Para a maioria dos magos, isso significa que a varinha deles se torna mais, e não menos, adequada a eles à medida que crescem.

“The best results, however, must always come where there is the strongest affinity between wizard and wand. These connections are complex. An initial attraction, and then a mutual quest for experience, the wand learning from the wizard, the wizard from the wand.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 24 (The Wandmaker)}}