To change the texsize, you can use \tiny, \small or other sizes. No need to use environments for this here, as the table-environment would group the fontsize to only that table, and not effect the rest of your document. Other fontsizes, along with some information about them can be found in the post What point (pt) font size are \Large etc.?
When creating multiple columns of the same type, it is easier to write *{11}{l}instead of lllllllllll, and it is much easier to read it. You can of course combine the methods and write `c*{4}{l}rr.
Note that I have not input the numbers as represented in your image, I just copied the first line.
I would advice against using a table this way. I'd try to make it less wide, so that it doesn't have as much white space and could be typeset in portrait mode, remove vertical lines, possible split that data into multiple tables. Mico also suggests a differemt fontsize, along with reduced spacing between columns, and I agree. I have made these suggestions into an example. Further improvements could be made of course.
Output

Suggested output

Code without suggestions
\documentclass[11pt]{article}
\usepackage{lscape}
\usepackage{multirow}
\begin{document}
\begin{landscape}
\begin{table}[ht]
\centering
\tiny
\begin{tabular}{*{11}{|l}|}
\hline
\multicolumn{3}{|c|}{} & \multicolumn{2}{c|}{\bfseries Age} &\multicolumn{2}{c|}{\bfseries Gender} & \multicolumn{4}{|c|}{\bfseries Education}\\
\cline{4-11}
\multicolumn{3}{|c|}{}& Mean & SD & Male & Female & High school & Undergraduate & Master or higher & Other\\
\cline{6-11}
\multicolumn{3}{|c|}{}&& & Count & Count & Count & Count & Count & Count\\
\hline
1st party & High information& Noticable & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\\
cookies & density & Non-noticable & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\\
\cline{2-11}
& \multirow{2}{*}{Low information density}& Noticable & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\\
&& Non-noticable & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\\
\hline
1st/3rd party& High information & Noticable & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\\
cookies & density & Non-noticable & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\\
\cline{2-11}
& \multirow{2}{*}{Low information density}& Noticable & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\\
&& Non-noticable & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\\
\hline
\end{tabular}
\end{table}
\end{landscape}
\end{document}
Suggested code
\documentclass[11pt]{article}
\usepackage{lscape}
\usepackage{multirow}
\usepackage{caption}
\usepackage{booktabs}
\usepackage{threeparttable}
\newcommand{\minitab}[2][l]{\begin{tabular}{#1}#2\end{tabular}}
\newcommand{\tableHeader}[1]{{\bfseries #1}}
\begin{document}
\begin{table}[ht]
\centering
\setlength{\tabcolsep}{3pt}
\scriptsize
\caption{Number of students enjoying cookies}
\begin{tablenotes}
\item Abbreviations: HS: Highschool, U: Undergratuate, M+: Master degree or higher
\end{tablenotes}
\begin{tabular}{{3}{l}{8}{c}}
\addlinespace
\toprule
\multicolumn{3}{c}{} & \multicolumn{2}{c}{\tableHeader{Age}} &\multicolumn{2}{c}{\tableHeader{Gender}} & \multicolumn{4}{l}{\tableHeader{Education}}\
\cmidrule{4-11}
& && & & M & F & HS & U & M+ & O\
\cmidrule{6-11}
\tableHeader{Cookies} & \parbox[b][1ex]{2cm}{\tableHeader{Information Density}} & \tableHeader{Noticable} & \tableHeader{Mean} & \tableHeader{SD} & \multicolumn{5}{l}{\tableHeader{Number of students}} \
\midrule
\multirow{4}{}{\bfseries 1st party} & High & Yes & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\
& & No & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\
\addlinespace
& Low & Yes & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\
&& No & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\
\hline
\multirow{4}{}{\bfseries 1st/3rd party} & High & Yes & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\
& & No & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\
\addlinespace
& Low & Yes & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\
& & No & 35 & 12 & 12 & 17 & 9 & 11 & 9 & 0\
\bottomrule
\end{tabular}
\begin{tablenotes}
\item *Records may not be correct as it was reported that many students thought the survey was regarding consumable cookies, and not internet-cookies
\end{tablenotes}
\end{table}
\end{document}
11columns, you're specifying only6and try to use\multicolumn{8}{....}-- how should this work? None of thelcolumn type specifiers will give wrapping in the cells as is apparently needed inMaster degree or higher. And of course, the post is a fragment only, missing the underlying document and the packages – Aug 13 '16 at 10:20