Very close. You are right to use \b. The problem is that you also used ^ which means "start of string". The number is not located at the start of the string.

$text =~ /\b(\d+)/
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Update: By the way, you shouldn't use $1 unless you made sure the match succeeded.

if ($text =~ /\b(\d+)\b/) {
   print("Matched $1\n");
}
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Update: Other solutions:

# The first standalone number.
$text =~ /\b(\d+)\b/
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# By context.
$text =~ /(\d+) months? old/
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# The last number.
$text =~ /^.*\b(\d+)/
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