Below  is a chart of age ranges for normal development for the various allophones of /r/.  It is important to keep in mind that the typical development ranges according to the student and the way that the study was designed.

Prevocalic or consonantal /r/as in words such as red, run and race are acquired approximately around  5 years of age.

Initial /r/ blends develop around age 8, with the exception of the consonant clusters thr as in thread, spr as in spread, str as in string and skr as in scratch which develop at age 9 years.

Vocalic /r/ acquisition ranges from  age 6-7 years and prevocalic /r/ typically develops around age 5 years.

Share this chart with parents,and teachers.  I put this information in the staff lounge for teachers so they can know the normal acquisition age for these sounds to determine whether a referral is warranted.

You can use this data to determine whether a student is appropriate for enrollment for intervention.