I got Sugar at 17 months and Spike and Gracie both accepted him with no problem. I'd just follow your heart. If you love her and feel like it's meant to be them go for it. If you feel like you'll miss the puppy stages then wait. Good luck with your decision!
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226