i have one question..
in 2011 bitcoins per 50,000 to 500,000 we spent for nothing, just pranking and stuff...
if one person kept 50,000 bitcoins in his account from 2011 up till now,
would the bitcoins still be valid for transactions?
Yup. All bitcoins that are recorded in the block chain ledger are valid and may be spent. There are many cases of lost wallets and some btc are still lying around in there sitting and may not be recovered unless someone develops a way to recover them (brute-forcing, improbable at the time-being).