I'm 24,
If I get my 1st class medical Jan 1st 2015, its valid for 12 months for 1st class privileges, after Jan 31 2016 it goes to 3rd class privileges only for the remaining 48 months. There would be no 2nd class priviledges. (I can however get a 1st class medical and use 2nd class privileges for 12 months as well, but still it goes to 3rd class medical privileges after Jan 31st 2016).

I'm over 40
If I get my 1st class medical Jan 1st 2015, its valid for 6 months for 1st class privileges, 6 months it allows me to still use 2nd class privileges for the remaining 6 months (because 2nd class medicals privileges are valid for a full 12 months no matter how old you are) but my medical will revert to only 3rd class privileges after a full 12 months (Jan 1st 2016) and be valid for the remaining 12 months.

Once you are over 40 yes you also need the ekg exam. My AME said they will typically do a base line exam around 35.