The usability of PGP has been a subject of much study:
Why Johnny Can't Encrypt
Among the eleven participants who figured out how to encrypt, failure to understand the public key model was widespread. Seven participants (P1, P2, P7, P8, P9, P10 and P11) used only their own public keys to encrypt email to the team members. Of those seven, only P8 and P10 eventually succeeded in sending correctly encrypted email to the team members before the end of the 90 minute test session
Email encryption schemes generally require you to install a security certificate containing a private key on your computer and to give your contacts a public key so they can send you an encrypted message. According to the study, this seems to be too much of a barrier for most people.
I've tried mailvelope briefly. It simplifies things, reducing the difficulty participants had with managing public key technology in the paper I linked to.
SecureGmail (only for use with Gmail and Google Chrome) uses symmetric encryption and requires you to get the password to your correspondent through a (hopefully secure) side channel.
I think the field of doing this through a browser extension is still immature. For example, I see that until March of this year, mailvelope encrypted your email after you type it, allowing the webmail service (Google) to save the plaintext somewhere. What other growing pains will it experience?
To answer your basic question, if I was trying to get my parents to encrypt email, I would tell them to compose a message to me in a file, use a desktop encryption tool to encrypt the file and then send the file as an attachment. This still would require them to exchange keys before hand and I don't know how to make this easier.