If someone remained a toddler for an extended period would mental development be affected

Okay, this is a really weird question but I wanted a second opinion.

I’m writing a fantasy novel, there is a curse that makes it so if you “die” you can’t actually die and the age you died at is what you will be forever.

I had the concept of a character who died very young, 2-3, and has been toddler aged for about 2,000 years.

The question is this; how exactly would the mental development be changed realistically? Could this child be able to read and write or is the mind of a toddler not able to do that? How much of our learning capacity is based on age and how much is based on learning? What even is the psychology and mental capabilities of a toddler and how would that be affected by being alive for 2,000 years worth of memory and learning?

Yes I understand that this is a fantastical scenario and if it isn’t right for this subreddit I will delete but I write myself into a corner and now I need help.