The link above shows all the results of recent by elections and change in voting since when the by elections were last held. Real people real votes. No weighting simply all those who bothered to vote.
Yougov poll 18-24 age group LAB 39% CON 39% LD 14% RESPECT 4% GREEN 2% NAT 2% ACTUAL RESPONDENTS 90 WEIGHTED UP TO 221
— General Election (@UKELECTIONS2015) October 15, 2013
Now lets break that down further of the 90 respondents for that age group 33% said they would not vote or did not know who they would vote for. Now as You gov don't publish exact number I have had to do an educated guess upon the percentages they give.
so 90 respondents less the 30 who don't know / won't vote leaving 60 voters considering most local elections only get 15% and general elections are likely to see falling turnout 66% is high for a group most likely not to bother.
of those 60, 39% are to vote Labour and another 39% vote Conservative. Considering one voter in this range has an effect of 1.66% you can see how easy it is for Respect to get 4% simply needing 3 respondents.
Plus how did Yougov round up. as 1.6r x 23 = 38.3r so would round down to 38 or 1,6r x 24 = 39.9r so rounded to 40. How did Yougov get 39%
So a handful of young people either way and now you see why you have such violent swings. If a group of friends all said JEDI would this develop a spike?