From the main Chailey 1914-1918 site:
"There are a two Albert Mitchell / Hussars possibilities noted at The National Archives: 6561 (later 45282) Private Albert Mitchell and 8214 Private Albert Mitchell and further research is necessary to positively identify him."
At the time of writing this, I hadn't seen the medal cards for the two men. Now that I have done so, I am a little closer, but only on the supposition that Albert joined the army as a young man. I'll explain.
Army service numbers can provide enormous clues in helping determine when a man joined a particular regiment. I've studied these in depth and I have a separate army service numbers website.
We know that Albert was born in 188o. 6561 could have been issued to a 4th Hussars man at some point between 1902 and 1906 or in September/October 1910 by which time the numbering system for cavalry of the line had changed. Up until December 1906, cavalry regiments numbered individually by regiment; after that date they numbered by corps.
8214 can only have been issued when this corps re-numbering had taken place, and the number itself dates to January 1912. Albert would have been close to 32 years old at this time and I think it unlikely, albeit not beyond the realms of possibility, that he would have enlisted for a regular terms of enlistment which would have taken him up to the age of nearly 40 (assuming a 7&5 term of enlistment).
In the absence of more concrete evidence therefore, I'm going with the deduction that Albert was number 6561 and that he enlisted probably in 1902 or 1903 when he was 22 or 23 years old. If he is that man, he arrived in France on 27th August 1914.