How to enumerate 1; 2.1; 2.2; 3?

\documentclass{article}
\renewcommand{\labelenumii}{\theenumii}
\renewcommand{\theenumii}{\theenumi.\arabic{enumii}.}
\setlength{\leftmarginii}{1.8ex}

\begin{document}
\begin{enumerate}
\item Initials
\item Age
          \stepcounter{enumi}
      \begin{enumerate}
      \item Preliminary Condition
      \item Support
      \end{enumerate}
\item Imaging
\end{enumerate}
\end{document}

If you are willing to sacrifice automation entirely, then you could use custom labels throughout:

\documentclass{article}
\begin{document}
    \begin{enumerate}
        \item [1] Initials
        \item [2] Age
        \item [3.1] Preliminary Condition
        \item [3.2] Support
        \item [4] Imaging
    \end{enumerate}
\end{document}

Here is an automated version using enumitem:

\documentclass{article}

\usepackage{enumitem}

\setenumerate[1]{align=left,label=\arabic*}
\setenumerate[2]{before=\stepcounter{enumi},label*=.\arabic*,leftmargin=1.2em,align=left}

\begin{document}

\begin{enumerate}
  \item Initials
  \item Age
  \begin{enumerate}
    \item Preliminary Condition
    \item Support
  \end{enumerate}
  \item Imaging
\end{enumerate}

\end{document}