Alignment in multicolum

If a \multicolumn is wider than the columns it spans, all the extra width goes in the last column, so it's better to make all the columns wider so their total width covers the heading, also make the heading narrower by making it two row. If you use dcolumn you can control the column widths and get the numbers to align correctly.

Please always post complete documents showing all packages used, not just fragments.

\documentclass{article}

\usepackage{float,dcolumn,multirow,dcolumn,booktabs}
\newcolumntype{,}{D{,}{{}{,}}{4.3}}
\begin{document}

\begin{table}[H]
\caption{Statistic values of dementia epidemiology over the world.}
\begin{tabular}{c,,,,,}
\toprule
\multirow{3}{*}{Region} & \multicolumn{3}{c}{ Number of people with dementia} & \multicolumn{2}{c}{ Proportionate increases} \\
 & \multicolumn{3}{c}{(millions)} & \multicolumn{2}{c}{(\%)} \\
\multicolumn{1}{l}{}    &
\multicolumn{1}{c}{2010}   &
\multicolumn{1}{c}{2030}              &
\multicolumn{1}{c}{2050}&
\multicolumn{1}{c}{2010--2030}&
\multicolumn{1}{c}{2010--2050}               \\
\midrule
Asia                    & 15,94              & 33,04              & 60,92                & 107                     & 282                     \\
Europe                  & 9,95               & 13,95              & 18,65                & 40                      & 87                      \\
The Americas            & 7,82               & 14,78              & 27,08                & 89                      & 246                     \\
Africa                  & 1,86               & 3,92               & 8,74                 & 111                     & 370                     \\
\midrule
World                   & 35,56              & 65,69              & 115,38               & 85                      & 225                     \\
\bottomrule
\end{tabular}
\label{tab:stats}
\end{table}

\end{document}

Just for completeness an almost identical solution with the »siunitx« package.

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{geometry}
\usepackage{caption}
\usepackage{booktabs,multirow}
\usepackage{siunitx}
\sisetup{
  output-decimal-marker={,}
}

\begin{document}
  \begin{table}[!htb]
    \caption{Statistic values of dementia epidemiology over the world.}
    \label{tab:stats}
    \centering
    \begin{tabular}{
      l
      S[table-format=4.3]
      S[table-format=4.3]
      S[table-format=4.3]
      S[table-format=3.0]
      S[table-format=3.0]
    }\toprule
      \multirow{3}{*}[-0.75ex]{Region} &
      \multicolumn{3}{c}{Number of people with dementia} &
      \multicolumn{2}{c}{Proportionate increases} \\
                    &
      \multicolumn{3}{c}{(\textperiodcentered\num{e6})} &
      \multicolumn{2}{c}{(\si{\percent})} \\ \cmidrule(lr){2-4} \cmidrule(lr){5-6}
                    & {2010} & {2030} & {2050} & {2010--2030} & {2010--2050} \\ \midrule
      Asia          &  15,94 &  33,04 &  60,92 & 107          & 282          \\
      Europe        &   9,95 &  13,95 &  18,65 &  40          & 87           \\
      The Americas  &   7,82 &  14,78 &  27,08 &  89          & 246          \\
      Africa        &   1,86 &   3,92 &   8,74 & 111          & 370          \\ \midrule
      World         &  35,56 &  65,69 & 115,38 &  85          & 225          \\ \bottomrule
    \end{tabular}
  \end{table}
\end{document}

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