How to select all records that are 10 minutes within current timestamp in MySQL?

The most efficient way would be to compare your timestamp (and only timestamp, without using any functions on it) to an expression that can be calculated as constant for every row, this way mysql can use index defined on your timestamp column.

SELECT * FROM myTable
WHERE last_seen >= NOW() - INTERVAL 10 MINUTE

You can always try EXPLAIN SELECT ... to see if an index can be used to find rows satisfying your WHERE condition without the need to check every row in the table.