First of all, I'm aware of 51% attacks and how they work. Though, I was wondering, if Bitcoin and other blockchains allow appending cryptographically valid, new blocks to blocks of arbitrary age (if no age restriction is in place).
For example, could I simply append a new block to the genesis block or are there any other restrictions than just the validity of the block itself? And if this is the case, would the "Bitcoin network" really accept a completely new blockchain starting from the genesis block, if I really managed to make a blockchain with higher difficulty, than the current chain? (Like a 99,999% attack :) )