Sacrifice only a grown-up animal, unless it is difficult for you, in which case sacrifice a ram (of even less than a year, but more than six months' age).
قَالَ رَسُولُ اللَّهِ ﷺ لاَ تَذْبَحُوا إِلاَّ مُسِنَّةً إِلاَّ أَنْ يَعْسُرَ عَلَيْكُمْ فَتَذْبَحُوا جَذَعَةً مِنَ الضَّأْنِ