st: A linear fit within a child's record

["Siyam, Amani" <siyama@who.int>]

Dear Stata-users, 

Below is a segment I have of a child's record that includes 22 visits: 

  +---------------------------------------------------------------+ 
  | visit   sex   age   tage   weight    tweight   thresh    flag | 
  |---------------------------------------------------------------| 
  |    19     2   549    548     12.1       12.1        7       0 | 
  |    20     2   583    609    12.35          .        7       1 | 
  |    21     2   670    670    13.85      13.85        7       0 | 
  +---------------------------------------------------------------+ 

I need your help to structure a STATA code that would perform a linear fit within a child's record as follows: 

By child: 
        take the observed weight when flag==1, the weight[_n-1] and weight[_n+1]; 
        regress the 3 points weights on the 3 points age (age[_n], age[_n-1], age[_n+1]) 
        and return the beta-coefficient of that model 
        or 
        predict tweight for tage(=609) if flag==1 using that model 

Many thanks for your help in advance. 

All the best, 

Amani

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