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  

Messenger of Allah ﷺ led us in the 'Id prayer in Medina on the Day of Sacrifice. Some persons slaughtered their animals ahead of him under the impression that Allah's Apostle ﷺ had-already offered sacrifice. Thereupon Allah's Apostle ﷺ said: Those who had slaughtered their animals ahead of him should slaughter the other ones in their stead. And they should not sacrifice the animal before Messenger of Allah ﷺ had sacrificed (his animal)  

أَنَّهُ سَمِعَ جَابِرَ بْنَ عَبْدِ اللَّهِ يَقُولُ صَلَّى بِنَا النَّبِيُّ ﷺ يَوْمَ النَّحْرِ بِالْمَدِينَةِ فَتَقَدَّمَ رِجَالٌ فَنَحَرُوا وَظَنُّوا أَنَّ النَّبِيَّ ﷺ قَدْ نَحَرَ فَأَمَرَ النَّبِيُّ ﷺ مَنْ كَانَ نَحَرَ قَبْلَهُ أَنْ يُعِيدَ بِنَحْرٍ آخَرَ وَلاَ يَنْحَرُوا حَتَّى يَنْحَرَ النَّبِيُّ ﷺ  

You sacrifice it.  

أَنَّ رَسُولَ اللَّهِ ﷺ أَعْطَاهُ غَنَمًا يَقْسِمُهَا عَلَى أَصْحَابِهِ ضَحَايَا فَبَقِيَ عَتُودٌ فَذَكَرَهُ لِرَسُولِ اللَّهِ ﷺ فَقَالَ ضَحِّ بِهِ أَنْتَ قَالَ قُتَيْبَةُ عَلَى صَحَابَتِهِ  

Messenger of Allah ﷺ distributed sacrificial animals (amongst us for sacrificing them on 'Id al-Adha). So we sacrificed them. There fell to my lot a lamb of less than one year I said: Messenger of Allah, there has fallen to my lot a lamb (Jadha'a), whereupon he said: Sacrifice that.  

قَسَمَ رَسُولُ اللَّهِ ﷺ فِينَا ضَحَايَا فَأَصَابَنِي جَذَعٌ فَقُلْتُ يَا رَسُولَ اللَّهِ إِنَّهُ أَصَابَنِي جَذَعٌ فَقَالَ ضَحِّ بِهِ  

This hadith has been transmitted on the authority of 'Uqba b. 'Amir al-Juhan with a slight change of wording.  

أَخْبَرَهُ أَنَّ رَسُولَ اللَّهِ ﷺ قَسَمَ ضَحَايَا بَيْنَ أَصْحَابِهِ بِمِثْلِ مَعْنَاهُ