Worth a punt, £4 for a £75 voucher...
Yes, it's for over 50-s only. EDIT; YOU CAN APPLY IF YOU ARE 49-85.
Minimum monthly payment online is £4 (first payment free) and it seems you only have to pay it once, but might be wise to keep paying until you get your voucher!
Diarise a chase-up for your voucher one month after your first payment (first payment is free, so that's two months after taking the policy out).
Your contract terms state, "If you have chosen a welcome gift, it will be sent to you around one month from the day you pay your first premium. Because the first month’s premium is free, this may be two months into your policy."
EDIT; POLICY DOCUMENT JUST RECEIVED THRU POST STATES, "...... your £75 (voucher) will be sent out (17 days) after your first payment."
Will you get the money? Well, ultimately you can complain to the FCA if they don't pay, so safish I'd say. If SunLife don't pay, complain through their offical process first with threat of FCA.....
NB. The pre-contract blurb, which is INCORRECT! states, "Your gift card will be sent to you around one month from the day you pay your THIRD premium. Because the first month is free, this means it’ll be sent around FOUR months into your policy. If you take out a plan, you are entitled to one gift card only issued from SunLife. We reserve the right to change or withdraw this gift entitlement, or limit the total number of gifts issued per customer. We also reserve the right to not issue a gift to any customer who has previously held a SunLife policy and has subsequently cancelled that policy."
Update 23/2/17, just received letters for me and wife with Amazon gift codes for £75 each. Applied to our account and worked fine!