Hey there, supercool_kid. Welcome to the boards.
One of the coolest things I have ever seen in my whole life was an ultrasound of an embryo masturbating while still IN UTERO. No kidding. Even before we're born we touch our own bodies from a place of curiosity, pleasure, comfort, or all of the above. There's no such thing as being too young for that.
Something else to know is that there's no one way to masturbate, and what we do is all about what feels good to us. Putting fingers or other objects inside the vagina is one way some people masturbate, but actually a) it's not the most common way people with that anatomy do, and b) even for those with that anatomy, just going right to that often doesn't feel good for most folks. People who masturbate or have sex with partners that way tend to do other things that feel good first to the rest of their bodies and to other parts of the vulva, like to external parts of the clitoris.
It might also be too, given your age, that you still have a good deal of the membrane -- the hymen or vaginal corona -- that is around or over the vaginal opening from birth for most folks covering or semi-covering that opening. That membrane gradually wears away over time, but while it's in that process, it can take a little more effort to get inside the vagina and can feel like you're describing. You might find it useful to get to know your own anatomy better by having a look with a hand mirror or your phone (sharing off!)?
Have you been able to take a look around the site at some of the articles about anatomy and masturbation yet? If not, would you like me to link you up to some to get started?