Not sure why age matters? If your meaning because of risk you might need it in next 12 months then dont go for 2-5 years. If your thinking i f you leave this earth the bond will be repaid to your estate but maybe not until it expires I dont know
How long for? If its say 2 years what s the chances you'd need the money before then??? If your trying to invest to build up money to pass on then yes go for a good rate, which will be a fix. But at 84 unless I was rolling in money I'd be spending/drawing down on it if i might need it- If I had for example say £10000 spare at 84 i'd rather draw down £2000 a year for 5 years than shove it away just to get £400 a year interest